Thats pretty much the same reason I wanted this.
Like I said, there has been quite a lot of discussions about this mainly on the request feature subforum, but there is no way to get a envelope control knob in either the TCP or MCP :/
The only way would be to have the trim volume envelope shown beneath the track and use that volume knob, but it gets really messy in larger projects. The only way to "pin" plug-ins to different placements in the chain is probably this workaround (that really isn't a workaround):
https://www.youtube.com/watch?v=p1RDUFxsz0Q
The best alternative I've found to having my faders roughly at 0dB is to gainstage with the item volume, either manually or through SWS Loudness, and then making sure that I don't let plug-ins that don't have automatic gain compensation increasing or decreasing my levels.
If you are using a lot of plug-ins that don't have AGC, you can use something like TBProaudio's automatic gain staging JSFX. It consists of one instance that you place first in the FX chain, and another instance that you place last. This will automatically compensate for any changes in level that any plug-in inbetween makes. There are other paid-for solutions like this as well, but the TBProaudio one is free.
None of these solutions are as clean as the trim knob in Cubase i.e, but there's not much else to do unless the Reaper devs decide to either add controls for the envelopes directly into the TCP/MCP or make it possible for scripts to add these controls.