Are OEM style plastic holders acceptable (for IVA purposes) for brake pipes and/or fuel lines?

I.e. a little plastic block which is riveted/bolted to the chassis, and it allows two brake pipes (and often fuel lines as well) to clip in and hold

This sort of thing:

Brake pipe clips (double) | eBay

Yes, they're fine.

They need bigger holes than screws/rivets holding P-clips, as they usually have thier own securing tangs that go into the hole. Rather than drill a hole in the chassis for each clip, I fitted an ally strip held with just 3-4 rivets long the tunnel and drilled that to take the clips

little tip.

if production car manufacturers are doing it then its virtually 99.99999% ok for iva. after all the iva is just a watered down version of the European type approval used for production cars.