Heh, some 15 years ago I was maintaining/modifying some old code for Windows CE PDAs. I remember how truncated the API was.

For OP use case I think that C++ is not a bad option, but it would still require quite an amount of work and I am not convinced that so many PDA users would transform their PDAs into cold storage.
I mean if one has a lot of free time for hobby programming, yes, it can be a nice project, especially if he has a PDA for testing.

But for most users I think that's easier, faster and possibly better too to simply buy a HW.