Yes, it's possible for any map to be played rifles only.
The way weapon selection is set in this game... well, there's 2 ways...
1. The server admin disallows all other weapons except for rifles. This will unrank a server by the way, so no experience points for you (Thanks a lot TWI *roll eyes*). I wager that this is the main reason why you don't see any Rifles Only servers now, as no server admin wants to run an unranked server on a regular basis.
2. As part of the regular process of making a custom map, the custom map maker has to set which player classes and weapons are available for each team, so if he wanted to make a rifles only version of his map, he could just restrict the class selection to Riflemen.
As to whether or not we at |DD| will run a rifles only map (or maps) in our regular rotation, I wouldn't rule it out as we have done that for games in the past, but I don't think that it is likely for the time being. We'd need to see enough demand for it from the community (personally, I'm for it). If there is, then we'll consider giving it a try, but for now I think we need to make sure that CTF works with the game "as is" before we try mixing in any other custom styles.