Not sure about private places to launch in Lomond (but I'm sure some of the caravan / chalet places must have that facility). If you are at the south end of the loch the slip at Balmaha is excellent.
You will need to register the boat tho' - have a look here to download the forms. The most expensive bit os buying the sticky numbers! They can even sell you a suitbale sized plastic board to stick them to so you don't need to wreck your toobs with stickers..... Registering your Boat - The Complete Guide To Boating on Loch Lomond
If you do visit Lomod, it's not a big tow to lanch at Helensbrgh / Rhu and the Clyde is your proverbial oyster. Likewise if you are round Tarbet way, the tops of Loch Long / Goil / Fyne are not a huge tow and Oban is only an hour or so for some West coast cruising.... From the top of the loch Oban is about 20 mins closer....
If that sounds appealing post back for more launch site info!