People use foliage for WB because it reflects loads of IR (the green is fairly irrelevant) I've also heard of skin, paper, PTFE & concrete being used as WB targets for IR (all give subtly different results but are surprisingly similar to healthy foliage.No problem. Their spectrum charts are interesting and more so if one plays with the WB with some filters and seeing their effect on the camera's LCD.
I first thought the primary colors might be a good place to start (I gave up on Red since my camera absolutely hated that color for a WB.), but they cut the spectrum of other colors so much you get a sort of solarized image using them to set the WB. The Lee #139 Primary Green produces a wild red solarized image on the LCD if used for WB that was starting, but I found later much could be resolved once the RAW/ORF file was processed, but it took more time to do so. Seems best to use a filter that passes a bit more color than entirely blocking much of the surrounding spectrum. Some show more IR passage above 700nm in the charts in the free "Lee Filters Designers Edition" sample pack.
The comparison part I used ON1 Photo RAW 2020 and made a Preset which helped me to sort through various WB made from different filter colors. Not exact, but close enough to determine what I think might work best for me, and a bit faster than editing each IR photo.
I'm sure their are a lot of other colors in that Lee filter assortment that one may find useful for an IR camera's WB. Just takes time to dig through them all. I put four different greens into the four WB memory locations in the E-M1 and settled on what I thought looked good for me and somewhat easy to post process. I did the Ambers given it didn't like Red, and might try the Yellows and Purples next. Some of the Blues produce a really striking contrast too.
Given I live in the desert climate with little greenery at times (Like now when it's 110F outside!), the Lee #121 'Lee Green' seemed a good choice for the foliage WB that many seem to use.
Have you tried using a variable ND for WB? With it you can adjust the visible:NIR ratio from about 1:1 to 1:100+ It might give a nice wide range of options.
I tend to keep 2 different custom WB settings on my camera & simply switch between them to see which works best for any particular filter/day/subject.