That should not be a problem, you must select an 8051 with PCA (Programmable Counter Array) and you can do PWM on an 8051 and use the method I have posted above to generate DTMF.
In this case you have to generate tones by yourselves. Made timer (e.g. T0) which time is in base of every tone and with diferent multiplication of time you'll get diferent frequences.