It's not elegant...
%asset_position^eq:1:A: ^trim%%asset_position^eq:2:B: ^trim%%asset_position^eq:3:C: ^trim%%asset_position^eq:4:D: ^trim%....etc
and keep going. Not sure what you want to do once it gets over 26. Perhaps you can do ^modulo:26 first, then it would cycle through the alphabet again.
The ' ^trim' modifier is necessary. The space provides the FALSE option in the conditional (otherwise you get a heap of 0's), and the trim removes the space again (otherwise you end up with a heap of spaces).
I'm sure better minds than mine could come up with a brilliant regex or similar.
Note: I can't test this at the present so can't guarantee it would work, and I've also had issues using modifiers on the asset_position keyword before (basically they didn't work), but I think that has been fixed in later versions.