Excellent questions, with good answers. My two cents on the thought.
A common misconception in the airsoft world is what a DM is. In airsoft, people use the term for a sniper who uses an AEG as a Designated Marksman.
However, in the military, a Designated Marksman has little to do with what equipment you use, but rather, what your job is.
The easiest way to put it is, a Sniper could most easily be thought of in the classic military role, (for example: ghillie, bolt action rifle, sneaking through the jungle). A Designated Marksman would be more like a S.W.A.T. team sniper.
There is also a military role of Squad Designated Marksman, in which you are still a squad member in a the traditional infantry rifleman role, but you have some extra training that gets your eyes and rounds on target a little better.
I think you're talking about being a traditional sniper in the jungle ninja sense. In this case, you should definitely wear a ghillie. The purpose of a ghillie is to make you look like ANYTHING but a person. I think entire squads in woodsy airsoft games would be wise to put at least a little ghillie elements into their camo around their head and shoulders (the most common element that identifies your shape as human).
To the point:
Drop leg is now the traditional way for everyone to carry a pistol. However, many snipers in the real world who are stalking all ghillied up carry a pistol and spare magazines in a shoulder holster under their ghillie top.
I think a cross draw is a bad idea if you're going to be prone. Airsoft guns can be tough, but dragging it through the woods laying on top of it certainly won't prolong it's lifespan. It will also probably dig considerably into the ground and your body. I wouldn't like to wear one for this purpose, but hey, if you like it, you'll find a way to make it work.
If you think you'll need to immediately transition to your pistol (quickdraw), go with the drop leg. If you just want it in case you run out of ammo, the shoulder holster is a safe bet. Depending on your ghillie, it may or may not be difficult to get to. Just leave a button or two undone.
Whichever way you choose to carry your pistol would also be a wise way to carry your ammo. You can get a drop leg holster and a drop leg ammo pouch for your support side leg.
The side your shoulder holster pistol sits on could be balanced by dangling mags from that side too.
In your specific case, I'd recommend these pieces of gear for either option, depending on your budget.
Drop leg:
Ammo platform
http://www.tacticaltailor.com/index.asp?PageAction=VIEWPROD&ProdID=87
Holster:
http://www.tacticaltailor.com/index.asp?PageAction=VIEWPROD&ProdID=45
Pouches:
http://www.tacticaltailor.com/index.asp?PageAction=VIEWPROD&ProdID=133
http://www.tacticaltailor.com/index.asp?PageAction=VIEWPROD&ProdID=65
Or something similar. I would like this setup because you can use zip ties to mount the triple pistol mag pouch to the single mag shingle so they draw toward your back. Then you can get them out from the prone pretty easy AND it saves leg space. But play around, maybe you'll like it.
Shoulder holster:
https://tacticalassaultgearstore.com/index.asp?PageAction=VIEWCATS&Category=77
I was actually searching for a shoulder holster that has two molle panels on either side, but couldn't find it quickly (I loose patience for that stuff when I'm searching FOR someone else...lol). I know it's out there, you'll just have to search harder than me. It's essentially the same concept as the drop leg I showed you above, you just moved where the panels were...
You could consider using a butt stock mag pouch for your spare rifle mag. Then you could go with whatever more traditional shoulder holster you wanted. I saw Leapers ones for like $14.
Remember, those were only examples. Search around for something in your price range. You can always find airsoft knock-offs for cheap, but just like the rifles, you get what you pay for.
