It can be done
Hello,
It is possible to do almost anything with the mast in place that can be done with the mast on deck. It just takes longer, and involves more risk. How much longer and how much more risk is a matter of quality of procedure, and every detail matters.
In your case, let's start with that aft-led line acting as a temporary support. The block will see about 140% of the load on the line; do you have a block that is strong enough to do that, and still give a safety factor of at least 10:1? And is there some good reason to lead the line aft, when doing so adds to elasticity, compromising the efficiency of the temporary stay while complicating the setup? What sort of line did you have in mind? Do you know what the load on it could be? And why would you start at the masthead, when without a spreader to help you this will give you a staying angle of perhaps 7 degrees? A proper sling, attached about 3/4 of the way up, would give a better angle. And what are you going to attach to on deck that is reassuringly strong enough? The upper's chainplate is the obvious choice, but can you use it without fouling on the shroud? And have you considered that you need to slack the other upper, as well as the backstay and jibstay before setting up on the shroud you are detaching?
These are the kinds of things that have brought similar projects to grief. People have managed to do what you hope to do, without addressing these issues (and others) but others have not fared so well. And if you think the cost of taking the mast out is high, consider the possible consequences of doing the job in place. I would urge you proceed only if you had the help of someone competent, and by that I mean someone who has (a) done this kind of thing many times, and (b) doesn't dismiss the above concerns.
Fair leads,
Brion Toss
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