It is a nice idea to use 555 as watch dog. I have used it succ for 8751. Use 555 is astable mode(free running osc) with appro ON-OFF time period(one will used as reset pulse in case of uc hang and other as watch time period withinh which u have to reset timig cycl & capa chaging). DO the following
(a) Put an MOSET across timing capa
(b) Tie MOSFET gate to 51 chip pin using
ac coupling ie put 0.1 uf capa and tie
gate to gnd with high resi 22k or 47 k.
(c) just issue a narrow pulse on pin
connected to MOSFET gate periodically (time < w. time)
AC coupling is very important because when Uc hangs many time reset pin may be in a state where it will perma prolong timer cycle.
555 is very cheap & reliable & ready available and so can be a very good option for processors not having internal WD like 89c51 , 89c52.

Anil