Hi Elizabeth @ess77, I don't have much knowledge on Plaquenil but did find two articles that mention it's use in treating the conditions.

"As a treatment for sarcoidosis, the antimalarial drug hydroxychloroquine (Plaquenil®) is most ... Severe vasculitis can be associated with exudates that." -- SARCOIDOSIS TREATMENT GUIDELINES: https://www.stopsarcoidosis.org/wp-content/uploads/FSR-Physicians-Protocol1-1.pdf

"Antimalarials - Chloroquine and hydroxychloroquine have been frequently used in treatment of sarcoidosis based on early randomized trials that showed a long-term benefit with chloroquine (56). However, based on the better safety profile, hydroxychloroquine is most often preferred. The mechanisms of action for hydroxychloroquine are varied; it can interfere with antigen presentation, prevent T cell activation, inhibit toll-like receptor signaling, and reduce inflammatory cytokines by T cells and B cells (57). Hydroxychloroquine has been particularly useful in cutaneous disease, hypercalcemia, and in some cases of neurosarcoidosis (58–60). Although gastrointestinal side effects are commonly reported with use of hydroxychloroquine, they are generally mild and well-tolerated (61)." -- Treatment of Sarcoidosis: A Multidisciplinary Approach: https://www.frontiersin.org/articles/10.3389/fimmu.2020.545413/full