Although this might work for you, it should be mentioned that none of the manufacturers will recommend this. Over-pressurizing can cause serious damage, and knowing when you have surpassed that threshold is usually indicated by the pain associated with it.
I don't doubt that the manufacturers that sell ONLY gravity or low pressure nasal irrigation systems won't recommend more aggressive positive pressure irrigation. Perhaps they have good medical reasons.
Those manufacturers that sell positive pressure and/or pulsing nasal irrigation devices have some good reasons for recommending their techniques. Just for additional info, here are some studies to back up the efficacy of a particular positive pressure system:
Nasal Irrigation - Sinusitis - Medical Reports
Regarding the use of positive pressure nasal irrigation such as I've described, using some knowledge and common sense should help one avoid excesses which might be harmful.
Dave C