The installation of "highway pegs" can fulfil the need of some taller riders who would like to be able to extend their legs during longer cruises. These extra footrests can be mounted on the engine crash bar cover.
These used to be sold by the site "Motorcycle Larry" as "Highway Blades"; but it appears they are no longer available. Attached to the engine crash bars, the pegs could be easily folded in or out with the toe of a boot, and were designed to pivot backwards for safety. Because they were mounted fairly high, there was no danger of catching one in a turn. Similar items are now available from psr-usa.com - but they're not cheap!
When installing any footrest, always make sure that all fasteners are tight and secure. If locknuts are not used, it's worth using a thread locking compound like Loctite.
Bear in mind though that the ST1100 was not designed with a "laid-back" posture in mind; after all, it's not a Valkyrie or a Harley!