From what I remember, it seems anything you put on the outside of your anus, will travel on the inside. Vaseline may take a bit longer since it is very thick - but if you have it to where it is very slick and soft, sorta like almost a melted stick of butter, it should penetrate towards the inside direction of your anus like other slippy creams should.
Else, you can also get a very small plastic syringe or some other very thin applicator to apply from inside out.
I know that with diarrhea, we may not be expecting it to happen, so it may be tricky with timing on how and when you should apply, but in the case of the colonoscopy prep, there was no doubt that you could possibly be having more than 2 or 3 trips to the toilet and that most of your eliminations would be liquified.