Botan can use essentially arbitrary backends for cryptographic operations. It has been tested with both hardware and software based backends, but there are only limited numbers of each, due primarily to lack of access to further hardware cards (as well as the associated drivers/SDKs).
Generally speaking, adding support for a new engine is not difficult (it typically takes a few hundred lines of code), as long as the appropriate hardware, software, and documentation are made available. Ideally these could be donated (making it possible to continue to test and enhance the module over time), but a loan or even remote access might be acceptable as well. If you have an interest in getting your crypto accelerator supported by Botan, please send a mail to the developer list.
The engine implementations currently included in Botan are
I am especially interested in drivers for 'open' platforms, ie ones with freely available documentation, and, ideally, open source drivers. In particular, SSE2/Altivec optimizations could probably be done by anyone who has a recent x86 or PPC processor and knows (or wants to learn) some assembly.