I did, no problems whatsoever. I did do a bit of planning to make sure all necessary engine wiring was in place, using two firewall pass-throughs. I have an O-320 with EIS-4000 engine monitoring, nothing too fancy.
The only rule that applies is that engines don't change much other than getting more expensive. Avionics change a lot and on the experimental side at least there is enough competition to hold prices. I know more than one RV builder who spent two or more years with a very expensive panel sitting on the shelf going to waste, then found a new something he wish he had instead.
There is a boat load of work to be done once the engine is mounted. Get that behind you and then you can do all the fun avionics stuff. You can still do pre-wiring as a lot of it is panel independent. You can make some brand choices however and get that EMS package to install engine probes and such.
If your engine instruments are going to be standalone, i see no problem in installing those with the engine. But, I'd leave the avionics until afterwards.
