Thanks for your reply.
From your user guide:
Request
IFR Release at Uncontrolled Airport
<CallSign> is number one for departure runway <RunwayName> at <ICAOCode> request IFR Release
Departure, Center, Approach
This is the only phrase listed for this purpose. You might want to update the manual!
Calling for release on the DEL freq was heard correctly (with the phrase you mentioned) but was ignored. Once I figured out that I had to change to the APC freq it worked properly. At the (real-life) airport in question, we always call for release on DEL because APC can't hear us on the ground. Also, the DEL controller didn't indicate a frequency change, just "call for release when ready" so this was somewhat confusing.
That must be why SayIt did not offer the "request release" phrase - I was still on the DEL freq.
Anyway, thanks for clearing that up!