There is dedicated matrix scanning hardware in some of the '51s - albeit not of the usual sort, they are intended for laptops - see links in http://www.8052.com/forum/read.phtml?id=115348 and followup. You might get some inspiration from them at least.
Alternatively, there are '51 clones (e.g. the fatter LPC9xx's) which do have a sort of an external inpterrupt on a bunch of pins (called ekyboard interrupts); you connect the matrix output into these, pull down all the matrix input and if interrupt occurs, do the normal scan.

Of course, the most common way working on any '51 hardware is to do the scan in timer interrupt, as already said here...

Jan Waclawek