It does look like I've been hitting a machine limit.
On a machine with 12Gb of ram I successfully produced a 7000 page PDF with tables.

However when I try for the 70000 page version I get the following at 4gb of memory use :

GC Warning: Too close to address space limit: blacklisting ineffective

I'm using the Linux generic version.

Now all you need is a 64-bit machine, to break the 4GB barrier!

(Or more realistically, we need to cut the memory usage for large numbers of elements).

We have now released Prince 8.0, which does reduce the memory usage and may help with large documents. However, running a 64-bit operating system is still the only way to use more than 4Gb of memory for a single process.