Hentie, I have an old Vermont American model that I have not used in about 30 years, after I got a lathe. I used to do twists and open spirals on it. From what I remember, one could use a template, but the template follower is at the front of the router carriage.
I don’t see how it could be otherwise. If the follower were in the midline, where would you place the template? The workpiece is in the midline.
If you are worried that the workpiece diameter will be smaller than that of the template, because of the “pantograph” effect of having the follower at the front, you can compensate by adjusting the bit height, or by increasing the template diameter, but the calculation involved in doing so, is beyond me. Trial and error, in my ignorance.
I do remember having some dissatisfaction with the bit not being perpendicular to the workpiece throughout its travel, because the carriage pivots on the rear rail, and the tip of the bit therefore travels through an arc. This was only a major problem when cutting open spirals - the spirals landed up having a “rhomboid” cross-section. For surface decoration (e.g. diamonds cut with a twist and reverse twist) it was not that noticeable.
I had had a vague notion of making a parallelogram-based carriage to try and keep the bit travel perpendicular to the workpiece axis, but the model I have is very basic, with a fixed ratio of turn to travel, so probably not worth it.
Several years ago there was a thread on duplicator router lathes, one post of which linked to an interesting YouTube video.https://www.routerforums.com/jigs-fixtures/43239-homemade-router-copier.htmlI want to modify my router lathe so that I can add a template or item to copy, at the exact distance that the router is from the pivot point