Somehow I didn't see this. First I'll assume you know everything Reaper can already do.
Ok, where would this indicator live ?
Second, I'll assume you know most of what Reaper can do, but perhaps not this.
Using the sidecar mixer layout, keep it visible as an inspector type of view of your currently selected track. Then have all the parameters you wish to keep an eye on be visible as a parameter knob.
The easiest way to do this is via the envelope window of the track. Then those parameter knobs need to be visible on the MCP. Activate the envelopes for those parameters.
Turn on WRITE mode and hit play. The knobs will be red, indicating your parameters are latched. In the case of WRITE mode, that's everything that's activated and armed.
Set the track to something like Latch Preview. All the parameter knobs that are for parameters with activated and armed envelopes should change colour, which indicates they can be latched and written to if you want. Move those parameters. Those knobs should turn red.
Use the "actively writing" automation writing actions to write to the envelope lanes.
This all sounds complicated, but it literally takes seconds when you've practiced it.
Those parameter knobs are good indicators of which parameters are latched.
Here's a shot of what that can look like. I'm using the sidecare feature of the MCP to have a column of just the inserts, parameter knobs and sends across the whole screen.