@Nyoxi, there is no need in [ab]using of
Architecture field. Please, for the sake of correctness, don't introduce confusing differencies with
original .deb format. Anyway, you could just use your own field, say,
Kindle-Firmware-Version (or, maybe,
X-Kindle-Firmware-Version to never conflict with future official fields). It is totally legal:
Control files and their fields - User-defined fields.
BTW, if you want to prevent installation of old incompatible (maybe, unsupported) hacks onto new firmware version, then differrent package repos for different firmware versions is the most sane idea. Checking of relevant control file field could be an additional option (enabled by default, but overridable).
OK, as I see, OE writes into .ipk
control file only predefined set of fields and it doesn't support user-defined fields, so [ab]using of
Architecture field is your best try with standard OE classes. But, still, it's awful solution.
Did you really already setup OE fully for building of packages for Kindle and problem of differentiating packages for some firmware versions is your last barrier?