While the official docs make no mention of "proto ppp" yet, the ppp package itself seems to be providing basic support for plain PPP (i.e. no PPPoE or PPTP) connections in /lib/network/ppp.sh. However it is currently not working due to the fact that, in case of "proto ppp" being used, the name of the network interface to be created is passed to pppd, which causes errors (in the PPPoE case, the rp-pppoe plugin uses this parameter). Also there seems to be no way yet to set the serial interface pppd should run on.
a) Are there any plans by others to complete this feature?
b) Any suggestions on the "correct" way to implement it?
(Just specifying "option device /dev/tts/1" or similar is of no use, since the "device" parameter is overwritten in /lib/network/config.sh's scan_interfaces().)