https://lwn.net/Articles/833222/ refers.

Based on this it seems that the kernel has a mechanism to support pppoe bridging to l2tp.

We're running xl2tpd (with some patches I've already published on the project page) on our nodes already in LNS mode, but adding a LAC portion here is should be perfectly possible.

The question then becomes once pppd has received a connection, can we either by way of radius tag (not sure which tag would be appropriate, not seeing anything jump out when doing grep -i lac /usr/share/freeradius/dictionary*) or some other configuration (say lacmap) then kick over into LAC mode (ie, bridge between whatever incoming, pppoe and l2tp here).

Using radius would be preferred I reckon, as that allows us to control in a more fine-grained manner, however, lacmap may be simpler.

lacmap file may look something like:

realm lacname

And then somehow pppd needs to query xl2tp (or another l2tp daemon) to establish the relevant tunnel and obtain details for us to establish the bridge.

Not sure if this is viable, and insights would be appreciated. Will also dig further myself.