No the rules won't be clear because it's a trial with the intention of establishing a firm proposal.
You will be able to use an ankle roll.
You will be able to use a flat mat to keep your knee off the ground. This mat cannot offer support. It keeps your knee clean and protects it from objects in the ground.
I cannot define the mat as there are presently loads of different mats that would suffice. What will not be allowed is the use of a beanbag as a mat.
It is my view that if in the future the BFTA adopts this rule then it will provide mats for use.
I'm afraid I'm not seeing a problem. Personally I'll be trialling a roll and use no mat. If others want to use a mat then let's see their solutions.
Can you wear a knee pad?
It's not against the rules.
Does the use of a roll and a pad offer advantage?
Let's find out but is it any different to using a bag under your ankle whilst wearing a knee pad?