Secondary index entries for docs with missing key values

Unfortunately I can’t find a way to target this index. Do you know of any way that I can do that?
CREATE INDEX TestIndex ON Test(IFMISSING(IsSpecial, null))

I can get it to work using false instead of null but false doesn’t work nicely when it’s a non boolean property. And the queries are getting pretty funky. I was trying to get rid of the IS NOT MISSING parts, now I am running IFMISSING computations on the query and that cannot be a good thing.

All your help is really appreciated.