It's a tough thing to do -- particularly with Excel -- many spreadsheets include VBA code and other stuff that has dependencies on other parts of the Microsoft stack.
OOXML is a beastly specification, because it's not just a description of how to create a document or spreadsheet -- it includes artifacts to maintain compatibility with MS Office software from the early 90's.
VBA is the big beast - but here is a question from me. What will it take to implement VBA on top of Lua (or any other lightweight,embedded runtimes/VM). Hell, what will it take to implement VBA on top of the V8 engine [1] ? If one can achieve that, then you can pretty much build a spreadsheet as a browser plugin for 80% of the cases. For the rest 20% pathological cases - fine, reboot to windows.
Now, thats's an idea for a Kickstarter. This is the kind of thing that goes on to transform businesses in third world countries - Excel is what prevents the move to Linux more than ANYTHING else.
"According to Microsoft, Excel 2011 for Mac features a full port of the Windows Office VBA environment; in most cases, the OS X and Windows versions of VBA now mirror each other."
Not to mention the various versions of it (e.g ECMA vs ISO, and their revisions) which led to differing and incompatible implementations, numerous areas where things are ambiguous or undefined, and things that are defined by referencing the undisclosed DOC/XLS spec.
OOXML is a beastly specification, because it's not just a description of how to create a document or spreadsheet -- it includes artifacts to maintain compatibility with MS Office software from the early 90's.