Raise mast on 21 macgregor
We have a MAcGregor 22. I have seen the setup you are mentioning but never used it. I think what I saw was someone using a set of lines rigged under the spreaders to the chainplates. As long as they are inline with the mast step they shouldn''t need lengthend or shortend as the mast is raised. Also the guy showing me this bended a third line to the lines used as Temporary Shrounds to recover them once the mast was up.
It will take some practice to get it to work good on your boat. One of our problems is the standing rigging getting fetched up on other gear on the way up. It doesn''t seem to matter how careful we lay stuff out something will get hung. i.e. we usually have a problem with the shrouds getting cought under the "pop top" cabin top or the backstay around the mast crutch. It is always nice to have an extra hand to run around a clear this stuff.
Another small boat sailor who does mostly single handed sailing we know uses an extra long jib halyard to act as a temporary adjustable headstay as the mast goes up. With the temporary shrouds and using the Jib halyard you could let the mast go for a second and clear shrouds and the backstay. Don''t do this if your mast step doesn''t have a pin through the foot of the mast.