The Cyclades-PC300 Linux driver now supports Frame Relay. It's not officially released yet, as I still have to do some heavy load tests. The Tx performance on this driver is much improved, thanks to the sample code I got from Krzysztof Halasa (the generic HDLC driver maintainer). However, operation of multiple PC300 channels in the same system still deserves some attention and investigation.
Probably tomorrow I'll be able to go back to the investigation of the Dell PE2400 and the Cyclades Async Driver.