I wrote a small translation tool from WDQ to SPARQL, which can be seen here:
Currently it supports only one model of data and only subset of WDQ
syntax, but this can be extended. I wrote it just as PoC to see how hard
it would be (not too hard) and to see which kinds of queries would be
produced in SPARQL. Don't put too much trust on the exact names of the
properties and entities, they are just used as an example now, but
eventually (if the tool proves useful) will be replaced with real ones.
BTW, speaking of the worries about being able to produce "heavy" SPARQL
queries, WDQ has tree and web operators which can produce very expensive
queries, and so can simple OR clauses, since they require unions which
can be very expensive.
If you play with it and notice some syntax that is supposed to work (see
the supported list on the page) please tell me. Other
comments/thoughts/suggestions also welcome.