Colin it would be a bit of a waste of a good eyepiece. Secondly guitar strings are way too coarse - try optical fibre instead. Better still is a laser-engraved glass reticle, or strands of real spider web as used for filar micrometers 200 years ago - the beauty of this is it comes with natural self-adhesive and is elastic, so can be stretched and won't sag. A fork was the traditional way to collect and apply spider silk.
It's also a big help if the eyepiece has a red LED illuminator.
There are some guide eyepieces with 70-degree AFOV at
http://www.apm-telescopes.de/en/eyep...eyepieces.html and the prices are OK while the $AU is high.
I've just ordered the 20mm 70 degree beastie to use while calibrating the encoders on my mount.