Springstay location question

[mariner2k]

Ok, bearing in mind that the point of this exercise is to be able to fly a mizzen staysail. The double backs on the main can be dealt with when I change masts.
Assuming I can't move the lowers forward 4' (though I will check), I'd like to back up a bit.
My old ketch had a springstay opposed by running backstays on the mizzen cap. I know how you feel about springstays but could this be a do-able option?
Or settle with flying a small staysail from just below the jumpers, which would puts me around 20' from the highest deck.
Why would a builder design a ketch without the ability to use a staysail? I assume that there is a reason for that configuration.