Hi,
I've only done one 14 day Master Cleanse, but I started it with high blood pressure too. Like you, I was very concerned about doing a salt water flush so I used senna instead. I bought senna pods & made a tea with about 6 pods every morning & night (seep them for 3 to 5 minutes). The resulting BMs were about 6 - 7 hours later for me (with only minor urgency). The only concern with senna is that you may become dependant on it. I used it for 14 days and I did have a bit of trouble for 2 or 3 days once I finished the Master-Cleanse but then all was back to normal.
What I also found was that I had to stop taking my BP meds after a few days on the Master-Cleanse as my BP was lowered so much! I loved that result but I'm afraid it only lasted a couple of weeks for me - maybe my old food habits creeping in again? I've read here that hawthorn berries and cayenne may lower blood blood pressure (I'm testing them at the moment) - maybe that's something you could check out for yourself?
Good luck with your cleanse!