Tunnel.py and low bit-rates

In 3.03 and before, as opposed to CVS, one could add higher samples per
symbol values to pick_bitrate.py and operate at lower bitrates than 35k.
Currently, gmsk seems to be the only one that works via that method,
dqpsk and dbpsk complain about sbs_to_mm not having an array key for the
particular value of samples/symbol. Did it previously figure out
something that worked and now uses a lookup?

Similarly, even with gmsk which lets me go below 35k, I seem to have
little to no success even with enough signal to operate at 35k.

What can I do, or what can be done to operate in the neighborhood of
1KB/s throughput and require the lowest possible SNR?

I’ve added in some error correcting code, but have almost no success
below 35k even if the tunnel appears to set-up.