Well, as the J said, you can force-load a world-builder save from your mods .ini (look Charlemagne mod\scenario for reference) - if you want your mod only have a single, pre-created map anyway, it's exactly what you need.
But i think there might be other options as well. For example the Final Frontier mod only allows to take it's own map-script. I am not sure, how it is defined, but you might want to dig there. That mods ini has this:
Code:
; Always start in the standard era
ForceStandardEra = 1
; Read Game options from XML, not .ini
ForceGameOptions = 1
; Allow public maps to be used with this mod
AllowPublicMaps = 0
NoCustomScenario = 1
So if you are able to hunt down that XML file, maybe you get some insight. Probably in the GameInfo folder. (The Civ4GameOptionInfos.xml there has tags for <bDefault>, <bVisible> possibly allowing to fix and hide at least the checkbox-options).
For Civs and Leaders at least, there is the <bPlayable>/<bAiPlayeable> tag in the XML, so you can lock players and or AI out from taking them.
Also, if anything else fails, i suppose, you could just delete all the unwanted difficulties from the Civ4HandicapInfos.xml, leaving the player with only one option to choose from (you will have to hunt down some dead references, but it should work). Probably similar things can be done to the CIV4WorldInfo.xml for size, and possible CIV4GameSpeedInfo.xml (tho i seem to vaguely remember, that adding or removing gamespeeds might cause some trouble with culture).
You should however also consider, if forcing players to certain restrictions would be a good design choice for a mod - it sounds like getting trough a lot of work, only to make your "customers" mad, because not being able to play the game in the way they want