Is there such part that allows me to replace the outer chainring and bolt on a chain guard? The kind of riding I do rarely calls for the outer chainring so it might be practical to replace it with something that can keep my pants clean...

Sure. Very common in the MTB world, commonly referred to as "bash guards". Problem being that you have to have A: replaceable chainrings and B: Be able to get them in the BCD of the rings you have.

Some selections: https://www.jensonusa.com/search/?s=bashguard

I decided on my commuter/tourer that I didn't use the outer chainring at all. I took it off and ground the teeth off. Now I have a nice chain guard. This setup worked out well for me (and it was free).

Jerry H

What tools did you use and how long did it took you? I thought about that but grinding steel is probably gonna take a long time...

There is a Taiwanese manufacturer of some very nice CNC machined aluminum chain ring guards listed as "Driveline chain guard" on eBay. Mounting bolts and spacers are included; has notch to accommodate the pin on the adjacent chainring. I and some other cyclists on another board have been very pleased with them. Available various colors and in larger sizes than you can typically get for BMX (e.g. 53 tooth size). Currently listed example.

Cyclo cross racers often run single ring setups with say a 42 or 44t chainring . I got a 110 for the 38t one to use on My Rohloff bike
from https://www.cyclocrossworld.com/
have a highly polished one on the 52 36 t setup I made up with a 144 bcd type campag triple.. as a double..

Giro, can this guard be used in addtion to the regular outer chainring? I.e. with a triple crankset can it function as the fourth outermost ring? The spacers and the cutout suggests that this would be the case. Could there be a dependence on the crank in this?

There's one from Velo Orange that's a bit smaller (46 tooth max, I think?) that allows the use of two chainrings - just get taller bolts and stick it on

https://www.velo-orange.com/suchgu.html