Do non-rhotic (British) English speakers often insert a silent "r" when respelling certain words?

It's fairly common in British English to write the /ɑː/ sound as ar in respellings that are intended to indicate the pronunciation of words. You can see in the following discussion thread two other examples, "Marfia" for Mafia and "Sharday" for Sade:

• Mafia [pronunciation], WordReference.com Language Forums

I assume speakers who do this don't think of it as inserting a non-pronounced r any more than you or I would think of the spelling "ah" as inserting a non-pronounced h, or the spelling "aw" as inserting a non-pronounced "w". In nonrhotic accents such as most typical southern British English accents, ar when not followed by a vowel letter can be viewed as a digraph for /ɑː/ (or sometimes for /ə/ in final unstressed syllables, as the linked blog post notes, although -er is the more common spelling for final /ə/).