Has anyone ever called a delay of game foul on the visiting team for showing up late? If so, what is the rule reference?

I've never seen it called to start a game but the reference would be 9-8-1-g.

UC for the failure of a team to be ready to start EITHER half.

It is not a delay of game. It is Unsportsmanlike Conduct!
 

It is not a Delay of Game, raher it is Unsportsmanlike Conduct and it is charged to the Head Coach. (Remember two and he is gone.)
My advice would be to give them as much rope on this one as you possibly can, including going to the visiting team dressing room and getting the team to prevent this preventable penalty. The visiting team head coach should have been advised as to how long halftime in the pregame. If all else fails then call it.
Rule Reference is 9-8-1g

I should have clearer with the question. The visitors showed up late because the team bus was late. They did not arrive until 5 minutes AFTER the scheduled game time. Got off the bus asked for 15 minutes to warm up. They never left the field. Game started 20 minutes late.

I had this in a sub-varsity game last week. We didn't penalize the team.

If it wasn't such a nice day maybe I would have. ;)

No foul!
 

I would not penalize this. This is an event outside the control of the head coach. You were correct to give him all the warm up time he requested.
I would put it in the game report to your association only.

You can't (or shouldn't, at least) penalize a team for events outside their control. The only time I'd call this one at the beginning of a game was if the team was there, had plenty of time to warm up, and the coach simply refused repeated instructions to get ready to begin the game.

I don't see any penalty for a late bus---
but if the team is at the site, and has warmed up, etc.
if the team delays the kick-off (like a goofy coach thinking
he can "psych out" the other team) then it is USC !!
Same for delay of the 2nd half.
As always, use good judgement !!