Probably no problem as you say RR, the main thing is being aware of limitations. I put girder forks on my 49 A7 which is wrong I know but... they look good and they seem to handle nicely, gives that much more vintage look and feel. Someone mentioned to me that I stand to wreck the forks with using a sidecar especially if poorly set up and heavily loaded, so with that in mind I grease up the links more often and take extra care while riding. Any chance of a picture of your Avon car?
Nothing fancy as yet, still a work in progress, has a BSA hub centre, but missing the boot handle.
I know what you mean about girder rigids being pretty. A lot of prewar bikes pulled heavy chairs and had girders. The only issue would be that typically the frames may run slightly different rake angles. A lot of people dont really understand chairs, I must see if I can find a picture I've got showing a rig racing that has impressive fork flex!
please forgive the mess in the shed

Lets see a pic of the girder 47 please! wink2