Hi, Oyvind. How are ya? Glad to hear you're getting some clients.
I'd check out what bitflipper said if you are using Kontakt exclusively (I don't know if that works because I don't own it but bit's a smart dude).
However if I were in a predicament such as that, and since many synths do NOT have fine tuning here is what I would do based on all my wacky experiments using the minimal stuff I have/know how to use.
Reaper has a "Varispeed" fader that will adjust tempo and pitch. I'd load a stereo mixdown of the backer tracks (obviously something mixed to your personal taste to ease writing/tracking) into Reaper then adjust the varispeed fader until I got the backer tracks up to (or down to) A 440hz. Export that then bring it into SONAR (or whatever DAW/sequencer I was using) and do my writing/tracking.
Once I'm done I'd take those files and import them into the original Reaper project then set the varispeed fader back to 0.0 then bounce/export the newly pitch/time stretched tracks.
If done correctly (with the correct settings in Reaper) it should be fine. Of course the less stretching the better so if you are closer to a whole semitone down then do your varispeed maneuver to match the nearest semitone and then you can just set your keyboard/hardware up or down by the appropriate amount of semitones (which means less stretching in Reaper).
I am currently working off beds that were recorded at A445 (but the guits/bass tuned down a semitone) so I've set Guitar Rig's tuner to 445 and am just tracking it all (and will release it all) as is. I think it might add a little bit of extra uniqueness/quirkiness to the tracks. However there are absolutely no synths or other wackiness going onto the tracks so I don't have to worry about the situation you describe.
Cheers!