Dumb Sheaves

[osteoderm]

What sort of sail and rig do you have? What is the trouble with the dumbsheave? Properly drilled and faired, with correctly-sized line fairly led, dumbsheaves are great on small craft rigs; cheap, light, strong, and effective.
Many small boat rigs with freestanding masts and dumbsheaves lead the tail of the halyard to the stemhead as a sort of forestay, particularly on small gunter and lug rigs. Another alternative for a lug rig would be a cheekblock at the masthead, on the same side as the yard.
If you have a small marconi sail, you may find that the fall of the halyard aft of the mast (given your planned-for block-on-a-strop aloft) may well interfere with the luff of the sail, whether laced on or otherwise. Leading the halyard tail to a belay point other than to the mast step or partners will either cause a foul lead or an undue lateral strain.
A small sheave box and sheave at the masthead is what i suspect you'd really prefer. Depending on the size/shape of the plug at the head of your hollow mast, this isn't really such a hard thing to do; just be sure you have enough wood to work with up there. It's just a matter of opening up the dumbsheave hole a bit into a rectangular slot, and pinning a sheave in the opening. Since the sheave only needs clearance for line at the top, and given how small such sheaves can be easily had, the hole need not be very large at all.
i've made this same conversion on other hollow masts for somewhat larger boats, using two small sheaves instead of a single larger one; this leads the line as far fore and aft as possible (for that fair lead), while keeping the slot as small as possible in the vertical dimension.