Most homebrewers do it in their spare time, but we don't have the looming deadlines and marketing departments to answer to.
The PCE has been a real nightmare to work with. In order to get the most out of it, you have to have a deep understanding of assembly language. For us regular C coders, we have... HuC. And HuC is a mess... it's lacking in so many ways. Frozen Utopia has literally pushed HuC past its breaking point and not even come close to fully exploiting the PCE.
The PC-FX, on the other hand, is a completely different story. The toolchain is built using gcc as a base, with standard binutils, working in a Unix environment. Official tools for it were leaked out long ago, many of which can be used now. The compiler is efficient and generates very good code, unlike HuC which generates tons of bloat (macro soup = a bad time is had by all). trap15 and I are working on high-level functions for liberis to make it even more powerful (he's on SCSI, I'm on 7up), and when we get the M-JPEG encoder working, we can fully exploit the RAINBOW chip. The rest of the hardware was figured out and exposed long ago. The rest is up to the design prowess of the software developers. I can hire talent to create content, and my own coding skill is more than adequate. Furthermore, Charles MacDonald has also expressed interest in developing for the PC-FX. His skill with code and hardware hacking is legendary.
Again, the PC-FX was never fully exploited during its commercial days for two main reasons: 1, the system did not have enough market time for developers to really dig deep, and 2, the industry was already well into the "marketing team rules" phase, meaning that designers were at the mercy of the almighty yen. The first issue only applies to those of us who are still relatively new to the system; the second detail will never apply so it's not a roadblock.
So yeah... I think it's totally possible for a homebrew developer to fully exploit the PC-FX. But then again, how many consoles were truly "fully exploited" in the past? And how do you really define "fully exploited"? Some people look to Dracula X as an example on the PCE... but there's a few things I would have done differently that would have made it even better (mainly in the data loading scheme... it's really poor here).