it seems there is nothing special method to implement what u want.
first,u can use "big_to_bytes" procedure to output the hex value to a buffer , then use "wsprintf" to convert to ASCII Character.
Second, u can modify the miracl lib by your self. find the "cotstr" procedure in mrio2.c,
trace to miro1.c , u will find itl call "otstr" procedure. so u can modify the source codes
from here

forever
{
nd=numdig(_MIPP_ mr_mip->w6);
...
}

and re-make ms32.lib by yourself. but this will change another proc of miracl,so be carefully.