Yeah I understand, but I wouldn't release all the ferts that are packed into the root tab into the water column, which is what I was alluding to. Since crypts are mainly root feeders, it makes sense to stick one in the driftwood, but exposing it to the water column seems to be a big no-no based on what you said as it releases too many nutrients at once.
Still, you could break apart the tab, grab a couple of the balls and jam them in there, it won't release as many nutrients into the water and could still feed your crypt.
I don't have experience with root feeders on driftwood, as I plant them in the substrate so I can use root tabs or clay fert balls. Perhaps liquid ferts can handle the needs if you spot dose the roots with small amounts regularly?