score:2
As no one answered this and I only got a weird comment that Romania in winter is not nice I think I should post the route and the result:
My Indonesian gf had a double-entry Schengen visa and arrived in Austria. We went to Szeged first (no problem), then to Temeschburg (Romania) - they looked a little funny at her at the border, took finger-prints, but everything fine.
Then to Novi Sad (the Romanians were a little confused, they never had any Indonesian traveling by car there, but everything was fine).
Serbia was no problem at all. Serbia to Croatia was fine too, no problem.
Croatia to Slovenia is only a problem if you cross at Hum Na Sutli, 7km away in Lupinjak it's ok. Hum Na Sutli only allows Schengen-Passport-Holders and Croatians to cross.