I'm sure a few extra days won't do any harm - just don't let them go so long they get root bound - once they are I've found poor recovery so plants just seem reluctant t push out new roots (you should try and tease them out rather than leave them in a ball).
@Oldbie I had to abandon my woodland site this year, which is one I had been slowly adapting in the autumn - tree thining | | (that's the height of sun v's tree height dance). Was a real find or so I thought till I saw a fresh flurry of horse hooves thru the clearing (a new occurrence) - damn I might have created a path.... shite ! ! !. An operation earlier this year killed the idea anyway so i'll never known if it wood have worked, but at least I haven't had to carry rucksacks of compost and boxes with plants in em by public transport (bus). As urbanites the only other green spaces we have around here are called parks haha.
Ha ha, No Spliffy, you don't want to be transporting site materials by bus if you can help it! certainly not 4 x 4 bits of plywood and rolls of chicken wire, not to mention transplant ready seedlings. the one bit of advice I think that sticks out from my experience for guerrilla growing, is don't use expensive seed; do a seed run at home, or somewhere safe first and then its not such a heartbreak if they get eaten by slugs or mice or pigeons or.... the list goes on...plenty can go wrong that's for sure.
bye for now matie.