You can easily attach blocks to the mast or the deck. From a practical standpoint you can do anything you want as long as you do it right and make it secure. The only question I would have is that it appears that you have a very nice, clean layout already. Think carefully about whether or not you want to disturb the aesthetic that Cal put into the boat. If it doesn't matter to you, then lead your lines back to the cockpit. What make is your mast?
This is my Starwind 27 with a Kenyon MORC mast (please pay no attention to the birdshit). The boat originally had 2 Kenyon halyard blocks mounted at the base of the mast for the main halyard and the genoa halyard (both internal with exits above the boom). In this case, the boat has a pad for the deck organizers and the original had just a pair of 2-sheave organizers (Schaefer). The only control line that came back was the vang. The reefing lines and outhaul were clutched at the gooseneck.
I said, "F-that", I need everything and more going back to the cockpit. I added 2 Kenyon blocks (from Rig Rite - very expensive), and 3 Schaefer blocks to the mast. They are all attached with tapped holes and SS machine screws (as done originally - no pointy ended screws). I also added 2 Schaefer deck blocks. To lead it all back to the cockpit, I drilled another hole for the deck organizer on both sides and mounted a pair of double-decked triples. So the boat now has 2 genoa halyards, 1 spinnaker halyard, main halyard, 2 topping lifts, 2 reefing lines, double-ended vang control, and the outhaul all coming back to the cockpit. There is one slot in the organizer on one side that is open for something else.
None of this compromises the mast or the deck in any way. But I will admit that if I were to do it over, I would use that part that
@mermike references.
If you don't want an excessive number of lines cluttering your cockpit, you don't need to go to that extreme.
I found that the machine screws are fine in the wall of the mast, as long as the holes are carefully tapped. The SS steel screws WILL fuse to the aluminum, I had to drill one out of the original installation. If the thread doesn't tap properly, the blocks are at the base and you can easily use a washer and a locking nut for any screw that strips the aluminum at the thread. But you have to wait until the mast is down. I didn't have any problem waiting a few years to fix one of those. Three out of four screws firmly secured did just fine at one block!
The parts are still expensive, any way you go about it. If you want the base plate instead of all the penetrations, I wouldn't hesitate to install the base plate. $750 to take the mast down isn't that much in the grand scheme of it all. Find other reasons to have the mast down if the cost still sticks in your craw!
If you are running lines back to the cockpit, you will have to plan the location of the organizers. My Catalina runs the lines direct from the mast base to the cockpit on a diagonal. It has a mast plate that holds blocks and also a few deck-mounted blocks.
Nobody says that you can't adapt your boat the way you want it. Some may object that it isn't "original", but it's your boat, your choice.
