A breeder friend of mine said that she learned about a research paper at a conference once that explained how to lengthen viability of eggs. I can't find the paper online, so this advice comes "take it or leave it"...
The research showed that if you incubated eggs for 30 minutes each week, you could extend viability all the way up to three weeks. So, it seems like you'd incubate 30 min at the end of week one, then again at the end of week two, then put them in the bator at the end of week three (or sooner).
Seems like she told me that the change in viability was comparable to the change between 7 day old and 10 day old eggs.
Again, since I can't find the paper, I can't guarantee results...