Some other things in MAME 0.254 that I didn't mention.

There's a preliminary driver for SMC1112 handhelds. Tiger's Quartz Game Clock and Double Wide Screen LCD handhelds are on this MCU, and probably more that we don't know about (Large Screen ones aren't). It has the double-screen version of Lucky Luke, and the quartz version of King Kong. No SVG screen for the LCD yet. I'm pretty sure several of the games share the same ROM and just have a different LCD.
driver: https://github.com/mamedev/mame/blob/master/src/mame/handheld/hh_smc1k.cpp
cpu: https://github.com/mamedev/mame/blob/master/src/devices/cpu/tms1000/smc1102.cpp

If anyone has one to donate to Sean (they need to be decapped, no known method yet to electronically dump them), send him an email, see top of page: https://seanriddle.com/

And I don't think the following ones fall under the definition of 'handheld game', but maybe other opinions differ:

Tiger K-2-8 aka Coleco Talking Teacher, was already in MAME but now it's working. It had an SMC1112 for the LCD.
driver: https://github.com/mamedev/mame/blob/master/src/mame/tiger/k28m2.cpp
swlist: https://github.com/mamedev/mame/blob/master/hash/k28m2.xml

2 talking hand puppets: Bingo Bear and Monkgomery Monkey. Sure, they're toys and literally 'hand held', but not games. You just move your hand and the thing talks random sentences. Kids could have conversations with it (and get bored quick...)
driver: https://github.com/mamedev/mame/blob/master/src/mame/tiger/bingobear.cpp
swlist: https://github.com/mamedev/mame/blob/master/hash/bingobear.xml

TI Maths Magiques, it talks French.
it's in this driver (same hw as Speak & Spell Compact): https://github.com/mamedev/mame/blob/master/src/mame/ti/snspellc.cpp