Thanks for the suggestion, I'll add it to our wishlist. Until then there are a couple of ways you might be able to achieve this. The easiest would be to duplicate the Generator and change the Limits parameters for each material ID you'd like it to support. It's not ideal because it creates a larger graph, but it would work fine.
Alternatively you could limit the segments instead of the generator using several Conditional operators, or By exporting their On/Off property and wiring an expression that tests for multiple IDs, For example:
if(XSplineMatID = 1 | XSplineMatID = 2 | XSplineMatID = 6 | XSplineMatID = 7,1,0)
Unfortunately this doesn't work for generators at the moment.
I hope that helps.