The code has to be monolithic - individually tokenised scripts will have the same problem.
If all subscripts are included in-line before tokenisation and you never need to expand variable names (variable-variables, indirection, Execute statements) then you could probably tokenise variable names, although I might have missed something.
I knew there was something else nagging me - the following will not work with tokenised variable names:
Code:
kix32.exe $sMyDomain="ACME" tokenised.kx
IMO the loss of these and other features far outweighs the very minor gains that tokenising variable names would give.
Perhaps tokenisation of variables names could be a tokeniser option for hardcore golfers 
Reducing variable names to single character can be handled by a pre-parser, if that is all that is required. 
