after you have dd-wrt on the router then flash a 'dd-wrt.v24_vpn_generic.bin'
The old WRT54G routers with 4MB flash, AFAIK are the only 4MB flash routers you will find dd-wrt ovpn firmware for.
I wouldn't use any build of this year or last year.
I would use a dd-wrt build from no later than 2017.... but that's just my preference for these very old routers
--- your VPN speed using a WRT54G router will likely be no greater than 8 mbs at best ---
Thanks for the thoughts everyone. We have the OpenVPN part on a Raspberry PI in the UK (server end - working fine), so if we can't flash the client-end router to support OpenVPN then I guess we'll be looking at a second Raspberry PI at the client end running the OpenVPN client and operating as a wireless access point too. Any final thoughts on the routers (or on the Raspberry PI route) much appreciated as always.