My tach cable is leaking oil pretty bad. Don't know why, & don't really care, because I've been wanting to finish my E to F gauge conversion for a while now & this is a good enough excuse to get started on the tach.

Anyway, my question is: Is it possible to plug the hole that the tach cable goes into on the engine? The F tachs are electric & need to be wired into the spark plug cable, so I just need something to plug the hole so it doesn't continue to leak oil. Any suggestions short of welding it shut?

Just looked at my spare GS head and I can suggest two easy options:

a.) Snug fitting hollow rubber cap with a closed end and some RTV sealant to seal against oil leaks.
b.) Hollow thread nut to screw into the threaded piece left behind when your remove the cable from the head.

Unless you remove the gear driven tach output piece from the head, something hollow has to be used to seal off the hole.
If you are a perfectionist, you could remove the whole tach gear setup from the head and use some JB weld or high temp, oil resistant epoxy to permanently seal off the output hole.

Some have used an O2 sensor bung.  I think OCD did it.

Bear with me, I don't own a GS. But does the engine side of the cable have a threaded sleeve with the cable running thru it? May be able to chop the original cable and just use that threaded sleeve to hold a plug over the hole. 

The 04 has a plug.

I took the gear out of the drive and siliconed a bolt with a nut in its place. Did this on several engines without any issues.