I don't really agree with it being used for some and not others. Either use it for all or don't use it. It only takes 1 game where a poor decision changes the course of a match and the unfairness is highlighted. I don't agree with the notion well it's been inconsistent for years anyway so what does it matter - inconsistency due to human fallacy is one thing, providing a technological advantage is entirely different. Using mobile VAR units at grounds that don't have GLT would also stop that technological advantage because VAR can be used for ball over the line or not queries.
As for housing all the VARs, I have said before that the FA should buy some of the mobile VAR vans. No need for the leased network line at clubs as the vans have a secure network that can be plugged in at whatever ground they are at - a more minor adaptation for clubs. They need a docking area, large enough for a long wheelbase transit and the panel is similar in appearance to what you'd see in a campsite for an RV to hook up to water/electricity. A few meters of hefty cables needed.
Can't be done for this season obviously but if the FA used a bit of foresight (I know, I know...) they could start preparing for next season!
They clearly can't do all FA Cup games using VAR so if going down the all or nothing route then it would be nothing. I wouldn't agree with that - do we want a Thierry Henry type handball to occur in the last minute of the FA Cup final and not be spotted because we don't have VAR?
I'd agree it would be worthwhile looking into ways in which the use of VAR could be enhanced, such as using mobile units, although I appreciate the restraints on resources the FA faces and there's probably a reasonable question as to whether the money could be better spent elsewhere.