Some research is ongoing:
root@Nokia 8110 4G:/data/local/tmp # tinymix | grep REC
468 BOOL 1 MultiMedia8 Mixer VOC_REC_DL Off
469 BOOL 1 MultiMedia8 Mixer VOC_REC_UL Off
490 BOOL 1 MultiMedia4 Mixer VOC_REC_DL Off
491 BOOL 1 MultiMedia4 Mixer VOC_REC_UL Off
509 BOOL 1 MultiMedia1 Mixer VOC_REC_DL Off
510 BOOL 1 MultiMedia1 Mixer VOC_REC_UL Off
root@Nokia 8110 4G:/data/local/tmp # tinymix 509 1
root@Nokia 8110 4G:/data/local/tmp # tinymix 510 1
root@Nokia 8110 4G:/data/local/tmp # tinymix 468 1
root@Nokia 8110 4G:/data/local/tmp # tinymix 469 1
root@Nokia 8110 4G:/data/local/tmp # tinymix 490 1
root@Nokia 8110 4G:/data/local/tmp # tinymix 491 1
root@Nokia 8110 4G:/data/local/tmp # tinymix | grep REC
468 BOOL 1 MultiMedia8 Mixer VOC_REC_DL On
469 BOOL 1 MultiMedia8 Mixer VOC_REC_UL On
490 BOOL 1 MultiMedia4 Mixer VOC_REC_DL On
491 BOOL 1 MultiMedia4 Mixer VOC_REC_UL On
509 BOOL 1 MultiMedia1 Mixer VOC_REC_DL On
510 BOOL 1 MultiMedia1 Mixer VOC_REC_UL On
And then, after turning on these parameters, you can issue this during the ongoing call:
root@Nokia 8110 4G:/data/local/tmp # tinycap lol.wav
And get a clear recording of both microphone and speaker.
P.S. Getting a working frontend for tinymix/tinycap interoperability is another story... :) But the capability is there definitely.