How difficult is it to design a minimal ARM environment?
AFAIK (and remember I haven't got a board made yet) it's not difficult, all the signals seem pretty straight forward. I am using a 4-layer board with power and ground plains mostly because it seems like a good idea and makes the layout a LOT easier. Whether or not it's required I don't know.
Also, how forgiving of questionable layout are these, at ~100MHz?
100 MHz is not fast enough that RF theory is needed,
Bear in mind that there really isn't any 100MHz stuff outside the chip, I have paid special attention to the USB and microSD signals. Also external memory is on the backplane so it's a fair guess that it won't run at full speed.
Most other signals are normal IO which shouldn't be too hard to handle. The SAM also has on-board terminating resistors on all IO pins.
I really don't think there's a lot of room in the market for something like this, though.
Quite possibly, I've always done things for the fun of it so that doesn't really matter to me.
That said AFAIK none of those board have this combination of features, whether or not these features are of interest to people is another story