Well that's not a welcome outcome for sure, but your news about rgn16 not dumping triggered a memory of that happening many years ago when i tried to dump the logo regions from some StreetPilot 2xx0 models. They're similar to and from around the same era as the 3 x GPSMAP devices you have.

Likely 16/0x10 is protected from copying in all the devices from that time, although i recall that i was able to flash a modified nuvi 3x0 logo.bin to a StreetPilot c3x0 so maybe they're all able to be written to, even if the region can't be dumped. Certainly, i remember being surprised that the SP c3x0 accepted the RGN flash and even more surprised when the image-modified logo file from the nuvi 3x0 actually worked in it. The SP c3xo is not related at all to your Aviation devices of course, however the SP 26x0 and 27x0 are similar hardware to them.

Another stirred memory from over a decade ago - the SP c3x0 and nuvi 3x0 logo images despite being displayed in Landscape on the device, are actually loaded into the BIN file in Portrait, BMP picture size for them being 240 x 320 x 16 pixels ... soooo ... just a long shot but if you loaded the BMP of your plane as a landscape using xImage but it uses Portrait like the 2 devices i mentioned above it might explain the problem of the missing splash image. Example below:
Spoiler: AU nuvi 3x0 splash
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