MESA_GL_VERSION_OVERRIDE with any value 4.6 or above seems to get these apps working- including half-life!performance isn't quite what i was expecting given native library passthrough (around 25-30fps in stable areas, stuttering to 15 in others), but still a great PoC either way!
i'm almost certain this isn't the right way to deal with this issue (the raspberry pi 4b most certainly does not support openGL 4.6 natively...), but thought i'd flag it up either way. thanks so much again for your help thus far! again, if you'd like me to bring any of this up as a github issue, i'd be more than happy to oblige.