I don't think the current system is broke, it works just fine and is fair and will be quite content if it stays in place. However I think my idea might be 'more' fair.
Lets take a C3/C4. The race will allow C5 mares or C5s with a claim to enter the race, but when it comes to doing the barrier draw they will be assessed as C5s and not C4s.

So say the order of draws is 10,6,9,8,7,5,4,3,2,1.

Under the current system the race draw could look like this:
Hairy McLairy C4 with a claim driver
Donaldsons Dairy C5 mare with claim driver
Hercules Morse C3
Bottomley Potts C4 mare
Muffin Mclay C16 Claimer $xxxx
Schnitel Von Crum C5 mare
Scarface Claw C6 mare with claim driver
Skin and Bone C5 with claim
Kids Book C5 mare
Tough Tom C4

So we have a position where Hairy McLairy, Donaldson Dairy, Hercules More & Bottomley Potts were all treated as C3s for the purpose of the draw. The rest were all treated as C4s.

Under my system, it would be what you really are. So a C5 can still stop back into the race, but will we assessed as a C5 for the purposes of the PBD.

The barrier draw could then look like this


Hercules Morse C3
Bottomley Potts C4
Tough Tom C4
Hairy McLairy C4
Schnitzel Von Krum C5
Scarface Claw C6
Kids Book C5
Skin and Bone C5
Donaldsons Dairy C5
Muffin McLay C16

As the race is a C3/C4 in this situation the the C4 mares/claim drivers don't actually mean anything. (If it was C3 only, the C4 mares/claim drivers would get into the field, but draw the back)
But it stills allows the C5 mares/claim drivers to drop back into the race - giving then an advantage. At the moment that is a double advantage imo (both getting into the race, and then the chance to draw better than lesser classed horses), and a disadvantage to the C3s who should be getting the best draw, not just a chance at it.
It brings it into par with PBD $ as well. It's not like they say Horse A (with a claim driver) has $60,000 prizemoney, but then take off the amount of its last win for the purposes of the draw. So why take off its last penalty for the purposes of the draw?

Not being an owner, trainer etc but it must peeve them off when a horse who has won more races a) drops back into the race but then b) gets a better draw.
The dropping back into the race is fine, it gives incentive for mares to race on and for junior drivers to get some experience. That's fine, but its just too big an advantage to them if they then take the prime barrier draws.

Thoughts?