First, it helps to understand why the limitation exists.
The RPZ mechanism has not changed in BIND 9.10. The documentation in
KB article AA-00525 (Building DNS Firewalls with Response Policy Zones
(RPZ)is still almost current. What has changed in BIND 9.10 is that it
is now possible to use as many as 32 separate RPZ files in a single
instance of BIND, and that BIND is not significantly slowed by such
heavy use of RPZ. Each one of those 32 policy zone files can specify
policy for as many different domains as necessary. The limit of 32 is
on the number of independently-specified policy collections and not
the number of zones for which they specify policy.
In earlier versions of BIND in which RPZ was implemented, having more
than one RPZ zone file required BIND to perform a separate lookup in
each policy zone to see if there was a match. In BIND 9.10, the policy
information is stored in a radix tree, in which simultaneous lookups
across all policy zones can be performed in sub-linear time that is
approximately proportional to the logarithm of the number of policy
statements in the largest collection (RPZ zone).
The improved implementation of RPZ for BIND 9.10 was provided by
Vernon Schryver and Paul Vixie. It is faster because it is O(log n) in
the size of the policy and because it can look up several items of
data in parallel. The new limit of 32 results from the use of a 32-bit
bitfield to identify the policy zones that affect a query. Previous
implementations of RPZ were O(n) rather than O(log n).
I've emphasized the relevant verbiage. The only way to change the limit of 32 is to update the algorithm to use a larger bitfield, or gut the optimization code entirely. Even if you were to double the size of the bitfield to 64 (or re-double to 128, etc.), you're still dealing with a static limitation imposed by the radix tree optimization. Since I'm not familiar with the innards of this algorithm, I also can't say how effective such an attempt would be to begin with.
You can get around this by using views that match your individual customers, which will give you 32 RPZ zones per customer, but that's as far as you're going to get without diving into the source code.