I have only used fully covered self-expanding metal stents for palliative situations such as malignant gastric outlet obstruction or duodenal perforation (by instruments or due to lymphoma). There are case reports of using fc-SEMS for benign conditions.
Your case is very challenging as the patient likely has recalcitrant duodenal obstruction due to medications (NSAIDS). I also suspect that there is a component of ischemic duodenopathy in the setting of sickle cell disease.
Your surgery team's concerns are quite important because if this duodenal disease-stenosis is indeed due to NSAIDs, the possibility of continued or surreptitious use of NSAIDs after surgery is real, and NSAID use may lead to giant anastomotic ulcers and failure of the Billroth II operation.
Your concept of using a fcSEMS as a remodeling tool is excellent and makes sense, expecting that the fibrotic lumen may expand some due to the presence of the stent.
My approach would be the following:
a) Evaluate the histology (and rule out ischemic duodenathy due to sickle cell disease).
b) Avoid all NSAIDs, referring patient to special pain management clinic.
c) Then would consider a fcSEMS, likely choosing one that can be delivered through-the-scope (e.g. Taewong). The proximal part should be attached to the stomach with a clip (e.g. over-the-scope clip). And I would leave the stent no longer than a few weeks, as the distal expande flange may lacerate the mucosa and induce more fibrosis (or even penetration).
d) The final decision to proceed should be backed-up by a multidisciplinary discussion with surgeons and the patient.