Hi BOAC,
I understand that beneath-the-wing engines produce a nose up pitch when you increase thrust (which has a negative effect on static longitudinal stability) and tha aft-fuselage-mounted engines produce a nose down pitch (which has a positive effect on static longitudinal stability).
Thus in both case, increasing thrust has an influence on the pitch and thus on longitudinal stability. However, according to my understanding, none is more influent than the other !