Did the microcode disassembly find any useful backdoors to read microcode without decapping?

Not really. The 386 does not have an interface to read the ROM direclty. Instead, it uses the Built-In Self-Test (BIST) to verify the ROM's contents. It's basically a checksum-like mechanism that verify the integrity of the CROM.

there are several iterations of 386 and only one was disassembled

so there's still a chance