IGh,
Not sure where you have been working but the majority of FT folks I've worked with have had fairly continuous careers - both inside & outside of the Lazy B, in industry & agencies. Most FT departments may have peaks with the new project du jour but they also have a pretty heavy sustaining testing load such is the longevity of programs & the ever-changing requirements (NextGen, fuel costs etc).
PF,
As you will see on previous posts, there are probably 20 engineers of all types working in Flight Test for every FTE who goes flying - and quite a few of those have at least a PPL or higher. Equally I know FTEs who have no license whatsoever. My advice is to try to join a Flight Test department in whatever capacity you can (which may be more dependent on the type of Mech Eng you are & your experience), learn the processes and eventually you may become an FTE.