I actually do this, myself, and have had no issues. Granted, I play mostly with friends and wouldn't bring that army to any tourney (not only because of the bases, and the gameplay issues, but nearly half of the army is non-GW figs) so if your situation is similar, I don't expect you'll come up against any trouble.
Tis better to sit in silence and be presumed a fool than to speak and remove all doubt.
Only problems are:
1. Deep strike formation.
2. Pivoting models to get a couple of millimetres extra charge distance.
To be honest, neither of these are particularly troublesome, as long as you don't try to get any advantage out of it. They are glued to the bases they were supplied with; there is no rule saying that those bases have to be rounded. Just explain to your opponent how you're going to deal with the above problems (i.e. concentric squares for deep strike, and always move/charge with a flat side of the base towards the enemy) before the game and you should be sweet.
As said before, mount regular sized and monstrous demons on round base, not square.
Movement trays can be built to accomodate those round bases in Fantasy Battle, and it gives the exact same space that square based large infantry would get.
That's what i did for my slaanesh legion, and it works wonders.
For steeds of slaanesh, there's a new "eliptic" base in 40k (for bikes i guess), so i mounted mine on the Fantasy rectangular bases and made mini-trays to put the model on for 40k games. You can basically cut a regular round base in two, and put it in front and back of the model (just need something like cardboard under it), and voila, your fantasy steed now has the exact same footprint as the 40k one.
Chariots are a bit more problematic, but by then...i think people will cut you some slack
Last edited by Oakwolf; 13-05-2012 at 11:51.
BloodBowl: Dark Elf, Vampires, Necromantics, Norse
LotR: Easterlings, Mordor, Rohan
Wh40k: Slaanesh demonic legion, Deathwing w.i.p.
WhFB: Slaanesh demonic legion
Mordheim: Undead, Marienburgers, Cult of Slaanesh, Pirates.
My demons, the nightmares of Antioch IV:
Want sexy demon on demon action?: