Basic Flash (Light Meter) Question

The best thing to do is to take an incident light reading off the subject and then simply set the background flash units manually to a higher setting (assuming that you have a flash meter). You haven't really described what flash units/lights you have, so it's difficult to give accurate advice. Ideally, you would be using flash for all of your lighting, in order to keep the colour accurate and not mix incandescent/halogen with flash, else you will need coloured cells on your flash to compensate. You should also be able to use flash without any ND filters, as they should be fully adjustable. If all else fails, with a digital camera, all you need to do is fire off a few test shots to determine the correct flash exposure for the subject and then repeat for the background, and you should be right in the ballpark.

 

It's really pretty easy in principle.
Your background exposure is determined by the camera settings only, so do whatever is necessary to underexpose by 2 f-stops while keeping your shutter speed at or below your sync speed. I would recommend manual mode unless the background lighting changes quickly.
Then set your key light to a best guess power level and set your flash meter to your camera settings. Pop the flash and get the reading. Depending on the meter it will either tell you how far off your exposure was or the correct f-stop to use.
Make the appropriate adjustment on the power level of the flash to zero it in and that's it.

Details:
The meter does not know about the ND filter. You can make the adjustment via the ISO setting as you mentioned or simply make the 4 f-stop adjustments in your head. For really accurate exposures you will also want to make an adjustment for the T value of the lens. Lenses with many elements or poor coating transmit less light than their f-stop value would seem to suggest. This is not an issue when using TTL metering, but again your external meter has no idea what lens you are using.
Telling the meter your shutter speed allows it to do a flash plus ambient reading which will be more accurate in terms of achieved exposure, but can mess with power adjustments. For example if the meter reads your exposure as being 1 f-stop too high then dialing down the flash by that amount might not be enough if there is a significant amount of ambient. In the extreme you could turn off the flash completely and still be overexposed.
RC mode issues:
If you want to use the cordless sync of the Seconics then you will have to use simple slave mode, none of the fancy modes will work. The Olympus system uses preflash communication which will start the Seconics trigger and will count towards the measured exposure even though the camera does not yet expose. In dumb mode all slaves (and the meter) will fire when the first flash happens. Radio triggers are usually easier to use than the light based control Olympus uses.
Rudy

 

The Olympus flash units can be remotely adjusted for power, but the Sekonic is most likely triggered by the optical pulse. I'd use one Olympus for fill flash on the subject and another for side light etc and the Sekonic for background lighting.

Radio flash controllers do work, but you have no control other than manual adjustment. Personally, the Olympus wireless is great for what you appear to want to do, but you have to learn how to use them effectively.

I have a Minolta light/flash meter, but with digital cameras, it's so easy to see your results and adjust accordingly, that I don't bother with it much at all. I've actually found it easier and faster to set up the lighting just using the camera. The ISO (ASA) reading on my meter don't really reflect what the camera will record, so that's one reason I tend not to use it.

 

Ok simple thing to get what you want is the Aperture is adjusted for the flash and the shutter for the ambient. So if you want to set the back ground 2 stops under you raise the shutter speed by 2 stops over the ambient reading, but in brighter light you run into under powered flash as an issue. There are a lot of guides to this sort of thing on the web, you could look at the strobist site for example. I have tried this with moderate success but only at dusk or dull days, flash units powered by AA batteries on the whole don't have the power. My solution was to mount a load of old flash guns that I had lying around with optical slaves to make a larger unit, it was very 'Heath Robinson' but works. If I was doing it seriously, over just having a go to see if, I would buy a portable high powered battery flash set (more like Studio flashes) so I had the power.

 

If you really want to defeat the sun, you need to do this: http://strobist.blogspot.com.au/2008/05/joe-mcnally-desert-shoot.html.

 

Thats the sort of thing, though 7 top of the range flashes is a fair amount of cash. I think you could pick up a portable studio light set up for less, learn to do it manually and you save a fortune, its not too hard with instant previews on cameras now.