It’s perfectly idiomatic, I would say. And a good addition to the options, one I at least hadn't thought of. How it relates to the It. idiom, I can’t say (and I don’t have time to read, carefully, the discussion you link, wienergriessler, but it seems primarily to concern differences in sense between the It. and the E. idioms).
There’s a distinction between my suggestions and #18, though. Mine suggest the somewhat paradoxical position that what is normally considered a virtue (namely being honest) is described as a flaw; so a degree of hard-boiled cynicism, some might just say realism, is involved. #18 simply says that’s she’s honest to an exaggerated degree, possibly with the implication that she’s doing herself a disservice in being so.
The " ... to a fault" idiom is used with numerous adjectives: "punctual to a fault," "hard-working to a fault," "careful to a fault," etc.