I don't play in UGC other than backing up and I am banned on their forums but I have a fundamental problem with UGC and their pre-season week isn't the map they pick, it's the fact they insist on having a pre-season. It is meaningless and it's existence is down only to "test" maps out, although much shittier and underdeveloped maps are thrown in the regular season anyway.
2 Problems I have found in UGC that are constantly repeating themselves every season.
1) Pre-season and playing in it, last Spring my team played Ginyu force, we elected to scrim Lakeside than play a terrible custom map that won't be used in the games that matter. Essentially a pre-season game is just a scrim, with no incentive to play, in fact you have more incentive to lose the match as it affects who you face early in the season, now you would rather play as many of the weaker teams than face a top side early on, that is my experience of it.
2) Divisional placements are a hot topic of dispute, it seems my experience of how you are determined your placement, especially as a new team is through judging your individual players previous experiences, which is of course no indication of how they will perform as in the past players who played "invite" or "Platinum" could have been playing this level at a time when skill level was less developed or they could have been in a particular role for their team although not a contributing factor to the reason their team was so good (A polite way of saying they were carried). The other way teams seem to be decided on their skill level is through whims of admins, often from their OWN scrimming experiences rather than the individual team.
Alternative
Instead of a pre-season match, why not do either a one-off playoff week for teams who want to play in either Platinum, Gold, Silver, Steel, Iron, Cotton, Helium, Californium league...etc. Give teams an opportunity to play for their place, if you're a long time team who are "High Gold", let them playoff vs a Low Platinum team. The only issue I can see with this is a lot of teams form with the intent to not play the highest level, some generally do want to sandbag lower opposition, but if you put them in a playoff and they don't play (or they play and they don't try) then you're well in your right to now allow them to participate where they ask to go.
When it comes to judging a teams' skill, you need to look at how they perform as a team and their is no better way of judging skill than teams playing high pressure playoff matches. Get rid of Pre-season, get some playoffs and get some meaningful games beforehand. It makes for good viewing, early season excitement and rewards teams who are serious.