There is something interesting about superimposing with TMS9129.
I am doing some tests with my PX-7 (pal version) these days.
I tried to mix (superimpose) an external NTSC video source and VDP seems to genlock fine with that. Result is that screen stretches up and down, so I think it is capable of output 60 Hz image but, interruptions seem to be weird, as MSX seems to (temporary) freeze while genlocking at 60 Hz.
MSX goes back to normal after disconnecting external NTSC video.
Maybe the case is the same for the ER-101. I think it is capable of superimose (and genlock) to 50 Hz but, in this case, as MSX only gets INTs from internal VDP, it keeps working in normal condition.
EDIT: ¿Is that Goldstar PAL or NTSC?
If NTSC, then maybe I am totally wrong 
Is that Goldstar PAL or NTSC?
It's a PAL MSX and it's works fine with the ER-101. As I said the video out of the MSX is not connected. LD player and ER-101 are both in 60hz, so there is no graphic glitch or logic bug. Games work like a charm.
Tested with Cosmo Circuit, Star Fighters, Badlands and Inter Stellar.
The Basic screen "generated" by the MSX is duplicated on ER-101. Ingame, graphics are only displayed by ER-101. ER-101 act like a 32x. 
