Thinking about the proliferation of STB's which are now rattling around in drawers since people have got new digital TV's, you are partway there with one of those.
I'm sure there must have been some 625 to 405 converters produced back in the day too, might be worth looking for. Then you would have a 2 box system without needing to invent anything. But where's the fun in that?
I know there were dual-standard sets, I dismantled a few when I was a kid! Possibly worth finding schematics for those for inspiration.
I was only taught about 625 in college and I've forgotten most of it since I never did electronics professionally. I would guess the conversion is mostly about throwing some lines away. Something along the lines (pardon the pun) of a sawtooth generator so the ramp allows 5 out of every 6 lines through, then the down-stroke is similar to flyback in principle and that line is lost.
How the z axis information differs between the systems I don't know. I have vague feeling bit of the 625 signal are negative going, where the 405 equivalents are positive?
(edit - with 5 out of 6 lines you get a bit of a fudge factor, but it's near enough considering the few lines that were given to ceefax etc anyway)