I would ask your gastro about
the Miralax prep. Its cheaper, over the counter. Your gastro shouldn't be adverse to you using it and if he/she is, he/she isn't up on the latest information on bowel preps and only concerned about
what is most convenient for HIM/HER and not his/her patients.
I haven't tried Miralax yet. Will be trying it next month. BUT - all who have tried it has said the taste is no where near so bad as the Lytely products, PhosphoSoda or Magnesium Citrate. (The last two require much less foul tasting liquid to drink but you still have to get down enough clear liquids to flush you out).
Your gastro has absolutely NO WAY OF KNOWING what prep you used. They only know if your colon was clear and clean or not.
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.