If you want to check that a flight plan is acceptable you can use the CFMU flight plan validator
here. This DOES NOT submit the flight plan but does check it for validity against the latest route database.
At least then you know the flight plan isnt going to be rejected because you have chosen some obscure airway that doesn't exist any more!
I agree also that Navbox does it quite nicely...just produce the plan and fax via your PC's fax/modem. Easy!
Fly safe...
Paul