Has anyone thought about rewriting the HP48 ROM? If Raymond is reading this -- perhaps incorporate Speed UI into a custom ROM. It has been a while since I worked on HP48 hardware (and software), but I imagine it would not require too much work given the right tools (sadhp to decompile the old ROM and then make adjustments to "update" to a custom ROM). We could even borrow ideas from the HP49/50 series (e.g. flash pointers).
I have designed PCBs for single-chip 512KB memory upgrades (128KB Port 0, 128KB Port 1, and 256KB Port 2), and could work on putting together a mini-daughter board with a switch to swap between built-in ROM and the new ROM.