Careful examination of the old o-ring found it about .004" undersize in crossection and there were some very small tears in the sealing surface visible under 30x. I had assumed that there would have been either a large amount of wear degradation or swelling, but the old o-ring had enough dimensional integrity to have sized a replacement (I believe a 2.4x17.3mm available at o-rings.com). I had originally been thrown off by the normal practice in o-ring world of sizing the o-ring by the barrel and groove sizes, but that approach did not yield anything close.
I read that Buna-n (the most likely material) is susceptible to atmospheric degradation, particularly from ozone. Might explain why my pumps always fail after a year-or-so of urban commuter riding, even with little use.
Last note- barrel corrosion and all that loose aluminum oxide seems to be a contributor the problem. The pump handle on the roadmorph latches securely so I now carry it upside down to keep rain out of the barrel.