Hmm, you know, I'm not really sure. The map would be embedded into the save file, but the thing is that a Civ V save file also includes what mods are in use for that save. So, even if the map embeds, the game may still think it requires a mod to load.

Hi Tazzik, thanks for replying - I ended up not bothering with custom map, as my main concern was that the teams started together.. I didn't realize there was that option when creating using all maps... I thought it was only on certain maps..

I know it's an old post but I have started a few custom games using custom maps from the steam workshop and it works well enough. Provided the mod states that it is multiplayers compatible and does not affect save games. You can find this info when clicking on the mod name while loading them.

The only problem I have found is that any city state placed by the modder will show on GMR as a ?? AI civ. If you leave the game on public access then you can be sure that a newbie will soon join as one of these city states.
This will crash the game for the last player before the offending newbie. You then have to wait until he leaves the game but most of the time those settlers with 0xp never read their messages.

Another problem just came up with a new scenario I started today. The scenario states 11 human players and 11 AI in teams of 2.
And it plays that way solo. You only have access to the 11 civs selected for human players. But when posted as a GMR scenario then all 22 civs become available. Of course I already have a couple of players selecting the wrong civs and I have to ask them to change.

If you get someone joining as one of the AI's, you could request an admin to kick them out of your game.
Either post here on the forums, in the "Chat only" game, or you could always send them a GMR message. I'm sure they'd be happy to help out.