Would it be possible to not just have a QA check based on the number of characters (and for example whether target segments are a certain percentage longer or shorter than the source segment, but instead to also allow the checking of the number of words in a sentence.

I realise that workarounds might exist - e.g. such checking the number of spaces in the target segment, using RegExes in the QA checker, although I sm not sure if you have anyway to count the number of instances of spaces in a target segment - other than comparing the number in the target to those in the source.