IO540,
In any half decent SEP, you might have problems contacting the ATCU from 10 minutes out
If you had a half decent radio you would have a range of over 30nm at 1000ft. Takes 10 minutes at 180Kt GS. Good speed for an SEP.
Of course the 10 minutes counts back from the time at which you want the clearance. You can still keep your dodgy radio and be willing to hold outside for up to 10 minutes (if they will give a clearance with such a radio - the receiver site could be the best part of 20nm from you when at the zone boundary in some cases!)
Incidentally I don't see why a flight plan necessarily comes into it. A FP is distributed only to departure, destination, and perhaps a regional info service. One might be crossing some Class D on the way but they won't get the FP
Check out the AIP for the times when a flight plan is required.
You can either plan ahead and ensure that the unit gets a copy of a filed flight plan if you made one (include their address in the required place) or pass the details (file a plan) by R/T.
Basically the system says that if your plan to transit controlled airspace you should file a flight plan prior to the flight. If you file in the air (pass the details to the ATC unit) you could be further delayed while the plan is processed.
Regards,
DFC