I use my Amex for daily spending, which means my Monzo round ups are few and far between.
Now that I can see my Amex transactions with Plus, it would be great if I could also round up these transactions. I would view this as working exactly the same as current round ups: moving money from my current account to a designated pot, though this may not work for everyone.
Wouldn’t this do the opposite though?
Say you have £100 in your account
Spend £72.84 on your Amex
You want to move the 16p from your account to your round-up pot? Leaving you with £99.84?
If you wanted to round up your credit card transactions wouldnt you technically not be saving the full amount you intend considering you would owe more to Amex and the interest calculated would be higher?
EDIT: I didn’t read properly 
I don’t quite see how that is the opposite, but yes pretty much. Except for the final step:
Have £100 in your account
Spend £72.84 on Amex
Move £0.16 from account to round-up pot, leaving me with £99.84
Pay (direct debit or such) £72.84 to Amex, leaving me with £27.00
Which, assuming a full payment direct debit (or a charge instead of credit card), works out exactly the same as paying directly from my Monzo account.
Yes, this is why I’d expect the round-up payment to come from my current account instead of the credit card (to avoid accruing interest on a cash payment).
In my previous experience some years ago, this was how Moneybox worked. They would observe your credit card then direct debit your current account for the round-ups. The downside of this was that it was really slow - the DD was once a week, when the round ups work much better if they’re quick. Monzo has the benefit of being able to make many tiny round-up transactions at no cost.