There is a little bit in the UK masonry code BS 5628. Extract below.
19.1.10 Natural stone masonry
The characteristic compressive strength of stone masonry with bed joints 10 mm thick or less, and in
mortar designation (iii) or stronger, may be taken as 0.35 times the mean compressive strength of the
natural stone masonry units or representative cubes when prepared in accordance with BS EN 771-6 and
tested in accordance with BS EN 772-1. Where the strength is derived from cubes, consideration should be
given to the direction of any bedding planes, see BS EN 771-6.
19.1.11 Random rubble masonry
The characteristic strength of random rubble masonry may be taken as 75% of the corresponding strength
of natural stone masonry built with similar materials. In the case of rubble masonry built with lime mortar,
the characteristic strength may be taken as one-half of that for masonry in mortar strength class M2
(designation (iv))
There is also BS 5390 Code of Practice for Stone Masonry, although this is more directed at materials and construction rather than design. There is also the old UK masonry code CP 111 which might have something.
I also think there might be a little guidance in the IStructE Manual For Design Of Plain Masonry Building Structures. I dont have my copy to hand so cant check.
Not much, but might give you some leads.