Should be plenty of support in that area, might get away with it longer than you think... not like a downtube, although this is probably one of the finest bodges ever:
Not quite 500k but I once finished my commute (from Camberwell to Putney) with a broken downtube secured with a toe clip strap and my scarf. Did the commute back home (to New Cross) with the same set up but reinforced with packaging brown tape from work.
Should be plenty of support in that area, might get away with it longer than you think... not like a downtube, although this is probably one of the finest bodges ever:
http://www.thedrumup.co.uk/cycling-riders/george-berwick-500km-on-a-broken-frame/