Question
Could you suggest any tips for shortening branch length and promoting growth further down stems on pinus mungo, mops and similar.

Answer
The problem with Mugho pines is as you have discovered.  Because of the way they are pruned and sheared in the nursery trade they become very dense on the outside of the growth perimiter, so dense that the lack of light suffocates and other wise starves out most interior growth.  If you want to develop interior branching quickly the only method open to you is to inarch graft to locations further down the branches or limbs you desire to develop.  This is usually beyond the beginer but it is an option.  The other method is to first thin out the branches you don't want for style purpose then cut back the branches you wish to develop.  In doing this you have to be careful that when you cut back a branch that there are some needles left on the branch or it will die.  This should be done in June if you live in the Northern hemisphere and December in the Southern.  This will cause new buds to form where there are some needles left.  Next season let these new buds elongate out till late June and cut back hard again, leaving needles on the branch to be developed.  You should start seeing new little buds forming on old wood farther in towards the trunk.  Any of these new little buds should be allowed to develop till they become strong.  Then you can repeat the process with them.  If allowed to develop strongly these new littl branches can replace the older one you have been cutting on for the last couple of years.  If you still want growth further in toward the trunk you repeat the process with the new branches you have developed.  This can take a few years, but everything in bonsai takes a few years.

Vance Wood.