Perhaps the WG has already considered this, but if not, might I suggest using an optional URI to identify stemming algorithms?

[170] "stemming" URILiteral? | ("no" "stemming")

e.g. 

for $b in /books/book
where $b/title contains text 
  ("dog" using stemming "http://www.example.org/porter") ftand "cat" 
return $b/author

where the URI literals recognised by an implementation would be implementation defined.

While the option extension syntax would allow an implementation to do something similar, since stemming is already part of the specification it would be useful to have this standardised.