I installed a mast plate this past spring, with the intentions of running some lines back to the cockpit. On my boat the main hatch is off-set to stbd, therefore, all of the lines will be on the port side. Will all of the blocks at the base of my deck steped mast, all pulling in about the same direction... um... pull the base of the mast out? The plate is screwed down throught the mast step into the deck with about 10 1 1/4" screws.
Ah... Isn't it a bit late to think about that?
As a practical matter, screws are good in shear--i.e. loads across the screw--but not so great in tension--loads along the length of the screw. Assuming you mast passes through the base plate rather than bearing upon in, the question then becomes what the screws are screwed into and whether that material will endure the prospective tension loads applied by you lines. The turning blocks through which the lines pass will apply a load to the base plate at an angle that bisects the angle made by the line as it comes down from the mast and turns around the block. If that turning angle is 90º, the load applied to the plate will be at a 45º and equal to the load in the line (tension) divided by the cosine of 45º. Assuming the plate is sufficiently rigid, the vertical loads will be carried by at least several of the screws you installed. If the plate is flexible, however, and it looks as it it might be, only the screws in the closest proximity to the turning block/line will be loaded (in withdrawal) which is undesirable. If your mast is deck-stepped with the step sitting atop the base plate, then the screws with endure far less withdrawal loading and, in shear, should be okay.
The saving grace to the foregoing (assuming a keel stepped mast) is that, on your boat, the loads on the lines may be sufficiently low that you'll get away with your lash-up. The only loads I can see you having to endure are the jib and main halyards and perhaps a cunningham. A vang lead should go the the mast itself rather than the base plate. Other likely loads, reefing lines etc. would only be applied when other line loads are somewhat reduced.
Hindsight is (almost) always 20/20. It's foresight one needs work on eh?