I suspect he'll struggle to find anywhere that meets those needs
Could maybe try Resipole Farm, they run the well know camping / caravanning site right on the shores of Sunart
http://www.resipole.co.uk/ as they are a farm I suspect they may have some space, but whether they would do it or not I've no idea.
Could also speak to Andy Jackson at Ardnamurchan Charters obviously he knows the area well, but as part of his business is hiring out boats he may not wish to encourage boat storage. Being the nice chap he is though I suspect he might well try and help.
http://www.west-scotland-marine.com/Contact.html
Could also try speaking to Marine Harvest, they have fish farm sites on the Loch, at least one of which is not operating. Big sheds, convenient slip etc, if approached nicely they may allow something of the sort - though I doubt it

But miracles have happened.
Other than that I would be tempted to contact the local estates, possibly Laudale, details from who owns Scotland
http://www.whoownsscotland.org.uk:
Estate: Laudale
Grid Reference NM 760580
Acreage 12816
County Argyll
OS 1:50k Sheet 49 Oban & East Mull
Owner Trustees of Keith Falconer 1993 Trust
Owner Address Braehead House
Braehead Drive
Barnton
EDINBURGH
EH4 6QW
Property Address Laudale House
Ardgour
FORT WILLIAM
PH33 7AF
If I think of anything else I'll let you know
