If you train enough with the meter then the RPE corresponding to a time/effort becomes second nature. Even I, as a schmuck hobbyist at this, have gotten to the point I can gauge it.
When I took my bike to France for work, I rode out to the mountains on the weekend to go sight-seeing and stuff. I guessed by RPE a few longer climbs I went after and it was fine.
Also, I've taken the cyclocross bike on the local hammer ride a few times without the meter and did just fine. If you're on front, you're on front. If you're not, you're not. If you're trying to break over a hill, you still need to hurt some boys.
The RPE has been sufficient each time.
In TT, if you're steady enough you can kind of look at it just once or twice and go by RPE also. Once you ramp into it and are going steady, it is what it's going to be. Maybe if there's a hill or something make sure you don't blow up.
My meter battery died right at the beginning of a 2 hour TT bike ride Saturday. Going by RPE only with the computer in my back pocket my HR was perfect steady the entire 2 hour ride. Which was the idea for that workout. Steady rise the first 10 minutes then didn't move for pretty much the rest of the 2 hours.