Have you tried boost.python ? Also, you may want to read http://www.boostpro.com/writing/bpl.html, as that seems exactly to be what you are aiming at.
No, I hadn't looked at this previously. But, you're right, the embedding functionality of Boost.Python seems to provide the functionality of eval in Python syntax.
As I see it the main advantage would be c++ like syntax
Seems like this the common request, so I'll think about changing the syntax (the parser is only a small part part of the library). Currently the library is dynamically typed, seemingly like Boost.Python. So, each time you call a script, different methods/functions might execute, depending on the arguments. It should be possible to use C++ like syntax instead; however, this would of course mean that the code would be statically typed. Also, whilst it might be possible to get mostly C++ like syntax, there would always be limitations. For example, the library could never instantiate "new" templates; e.g if you want a vector<int>, you'd have to register it, unless a polymorphic system like Java Generics were used instead (which can likely only work for simple C++ template classes).
There may be some use for it if was lightweight and easily integrated
Well, it's currently header only and non-intrusive; so relatively easy to incorporate. However, it's probably not that lightweight in terms of compile time, run time and executable size.
Could this eval be used, somehow, to help solve the problem mentioned
I don't think so, sorry. Boost.Spirit really needs to instantiate many "new" templates, afaik. I don't think it's possible to instantiate these templates from within the script (since it actually requires compiled code generation), unless you mention them all beforehand in the compiled program, which is probably impossible in this case. Christian