For native projects, CafeTran offers an excellent feature to Export and Import bilingual documents for external review.
You can either use Project > Export as bilingual document or Project > Export as bilingual document with notes.
If you use the latter, I recently noticed that changes to Notes are also mirrored in CafeTran when you import the file back.
Which is great, since the reviewer (or yourself) can edit or add notes that will conveniently be displayed in CafeTran.
This has information has been added to CafeTran Espresso - Menu and Interface reference document.
For Excel translation projects, the Notes column could also be used for pasting /importing context or other information from an additional spreadsheet column.