I was curious to know if you've ever come across Travels in Tartary, Thibet and China, 1844-1846, a two volume account of a pair of Lazarite missionaries, Evariste-Regis Huc and Joseph Gabet, who, if their account is reliable, were the first Catholic missionaries in Lhasa, Tibet. They did not, apparently, have Church sanction for being there.
From the copyright page of the one I have:
"This Dover edition, first published in 1987, is an unabridged republication in a single volume of the work as published in two volumes by George Routledge & Sons, Ltd., London, in 1928 in the series "The Broadway Travellers." [Original French publication 1850, as Souvenirs d'un voyage dans la Tartarie, le Thibet et la Chine pendant les annees 1844, 1845, et 1846. Translation by William Hazlitt (same title as present volume) first published 1851.]"
It's a good read and offers some interesting implications for any exploration of what might arise from an encounter between religions (and their apparently inextricably linked political/economic masters).