There is ${name//pattern/string} and ${name:pos:len} for strings but I haven't found any similar documentation for manipulating arrays.
So far I've just been using shift and/or unset for simple manipulation. The only other plausible alternative would be to loop over the values and reconstitute a new array but that isn't desirable.
mksh developer here ;-)
This is not yet implemented currently, to answer your question. It has been on the TODO for a while already, going as far as to be a comment in the source code file in question, so it will probably materialise some day (a good chance to add it is when we’ll have multi-dimensional and associative arrays added, which prompts a partial rewrite of the code in question anyway).