Due to some emergencies cropping up from time to time, I have had to do something similar, and have had no problems so far. It is probably not best practice, but if you have to do it, then so be it.
I would suggest that it is best to mash, sparge, then get the wort up to the boil, and then cube it (although I have left mine in the urn (wrapped up nicely with the lid on and blankets to hold in the heat as much as possible). You can then pour it back into the boiler and get it up to the boil, and start your hop additions. I have done this several times with no ill effects, but boiling is good to stop the enzyme activity in the first place, and secondly to sterilise the wort to a certain extent for a certain time.
The longer you leave it the more chance of there being a problem, but if it is boiled first then the lid is put on the boiler and it is left wrapped in a doonah, there is little chance of nasties getting in over the next 24 hours. The next boil will kill anything that *might* have grown in there in that time, and the hop additions of the boil wont hurt none either!
As I said, not best practice, but we can't all have the time to spare in one hit to brew either......
cheers,
Crundle