Rig a back-and-forth trolley run on my oval tracks?

You got it! What you propose is perfectly fine. Exactly the way to do it. Anything that makes it reverse by touching the bumper on the trolley hard enough to push it in will reverse it and no harm will be done.

Yes, perfectly feasible, and you can even make it much simpler. To achieve the same effect, all I did on my inner oval loop was screw a long deck screw through a small block of gray foam into the top of the layout next to the center rail at the midpoint of the trolley station. The foam aligns with the trolley bumper to cushion the impact, and the station is long enough to plausibly emulate a (very!) brief trolley stop from both directions.

[BTW, I've since converted to a bump-and-run version with two trolleys on that loop, with one trolley stopping at the station until the second trolley approaches from behind to release it, using old-fashioned insulated/isolated track sections. A bit finicky in operation, but much more plausibly prototypical IMHO!]

I have run my MTH bumper trolley cars with a sectional track mounting screw sticking up high enough to hit the bumpers and reverse the cars. You could use more scenic devices to reverse the cars such a pile of track ties or a dirt mound. Brad nails are difficult to see, so a brad sticking up at the end of track will work to reverse the trolley. I also have MTH Railking Brill trolleys cars that can be programed to reverse at the end of track.

By the way, I find that the MTH bumper cars can be run slower to reverse than Lionel cars.

Thanks to all for these replies!

I shall go forth.

Mannyrock

I made a temporary bumper and screwed it down between the rails. It's on the left end and the right is a bumper at the end of the siding.

Good ideas from @Mannyrock and all the responses. I've also got a layout with no room to add a separate run for my #60 trolley, but a removeable bumper can work.

I had no room on my layout for a trolley so I went up.

I have it running most of the length of one side of my layout on Lionel trestles (111). Just installed bumpers at the end.  Kind of an overhead rail through my city area.

And found out a giraffe car wont go under it.

Bob