And, yes, a Favor Obligation to a fellow PC which is, in and of itself, worth Obligation is a valid Obligation.
Excellent. We are finally making a little headway. OK, now...
Let's say, by your "standards", the Obligation needs to be "worthy". Whatever arbitrary benchmark you set, let's try one:
My PC owe's Joe's PC for saving his life. My PC was in a bad situation. A lowly slave toiling away on Tatooine. Being beaten ruthlessly every day by his cruel master, an Aqualish thug. My PC would have surely died eventually from the continued abuse. Joe's PC came along one day and killed the Aqualish, and rescued my PC. Now my PC owes him bigtime.
The simple question I am looking to have answered at this time:
Is that "Favor" enough for you that I can take it as a starting Obligation during character creation?
Not yet. So far, you've only described the circumstances by which the Obligation came into being. You *haven't* described the circumstances by which the Obligation could be considered 'repaid'.
If your PC has to protect the other PC for the rest of his life, then yes. (Note: This is equivalent to the Wookie Life Debt originally being discussed)
If your PC has to save the other PC from certain death or slavery, then yes. (And this is probably going to be easier to repay, because typical PCs end up in all sorts of life-threatening situations over the course of a campaign.)
If your PC has to shine the other PC's shoes, then no.
Without a reasonably specific means by which the potential Obligation could be 'paid down', which is in sync with the rank assigned to said potential Obligation, you don't actually have the full Obligation yet.