So it should actually work in the container if you run the command xpra
upgrade, like so:

container$ xpra start :100 # wait a few seconds for it to start
container$ xpra upgrade :100
container$ xpra list

Just starting it in a container isn't terribly useful though since that
way you can't attach to it. If you run
guix environment --ad-hoc xpra -C --share=/tmp/.X11-unix
--share=/home/$USER/.xpra

and then start the server with the above commands you should be
able to attach to it outside of the container, like:
$ xpra attach :100
$ DISPLAY=:100 gui-program # on a different terminal

Does this work for you too?

I'm not sure why the 'xpra upgrade' command seems to be required in a
container. I have no idea how to fix that.

On Mon, 26 Nov 2018 10:00:37 +0100
ludovic.courtes@inria.fr (Ludovic Courtès) wrote:

> Hi Rutger,
> 
> Rutger Helling <rhelling@mykolab.com> skribis:
> 
> > apologies for the delayed reaction, I've been quite busy. Oddly
> > enough the commands seem to work fine on GuixSD. Can you maybe tell
> > me what distro you're using?  
> 
> I’m using GuixSD.  :-)
> 
> If we use ‘-C’ instead of ‘--pure’, we should both get the same
> result:
> 
> --8<---------------cut here---------------start------------->8---
> $ guix environment --ad-hoc xpra -C 
> [env]# xpra start :100
> Warning: Xorg binary not found, assuming the wrapper is needed!
> 
> Warning: running as root
> [env]# Entering daemon mode; any further errors will be reported to:
>   /tmp/:100.log
> 
> [env]# xpra list
> Warning: Xorg binary not found, assuming the wrapper is needed!
> 
> Warning: running as root
> Found the following xpra sessions:
> /home/ludo/.xpra:
>         UNKNOWN session at :100
> Re-probing unknown sessions in: /home/ludo/.xpra
> 
>         UNKNOWN session at :100 (cleaned up)
> [env]# 
> [env]# xpra list
> Warning: Xorg binary not found, assuming the wrapper is needed!
> 
> Warning: running as root
> No xpra sessions found
> --8<---------------cut here---------------end--------------->8---
> 
> Same for you?
> 
> Thanks for your feedback,
> Ludo’.