<p>I recently upgraded a c# windows service to run as a 64 bit .net process. Normally, this would be trivial, but the system makes use of a 32-bit DLL written in C++. It is not an option to convert this DLL to 64 bit, so I wrapped the DLL in a separate 32 bit .net process and exposed a .net interface via remoting.</p>

<p>This is quite a reliable solution, but I would prefer to run the system as a single process. Is there any way I can load my 32 bit DLL into a 64 bit process and access it directly (perhaps through some sort of thunking layer)?</p>

No, you can't.
Both 16-bit and 32-bit Windows lived in a 32-bit linear address space. The terms 16 and 32 refer to the size of the offset relative to the selector.
...
First, notice that a full-sized 16-bit pointer and a 32-bit flat pointer are the same size. The value 0x0123:0x467 requires 32 bits, and wow, so too does a 32-bit pointer. This means that data structures containing pointers do not change size between their 16-bit and 32-bit counterparts. A very handy coincidence.
Neither of these two observations holds true for 32-bit to 64-bit thunking. The size of the pointer has changed, which means that converting a 32-bit structure to a 64-bit structure and vice versa changes the size of the structure
. And the 64-bit address space is four billion times larger than the 32-bit address space. If there is some memory in the 64-bit address space at offset 0x000006fb`01234567, 32-bit code will be unable to access it. It's not like you can build a temporary address window, because 32-bit flat code doesn't know about these temporary address windows; they abandoned selectors, remember?
http://blogs.msdn.com/oldnewthing/archive/2008/10/20/9006720.aspx