I think that the relation between Shoana and Joanna is clear.
1) Shoana seems an attempt to Anglicization of some variants of Joanna with this exact sound (the Galician Xoana, for example), just as Schoana is cited in many German sites and boards to suggest this pronunciation (German SCH is equal as English SH):
"Xoana* (*sprich Schoana, galizisch für Johanna)" and "Xoana* (*spoken Shoa:na:, galician for Joan)" (http://www.maaki-bears.de/newbears_Xoana.html
"Ich heiße: Joana (sprich Schoana)" (http://www.beepworld.de/cgi-bin/forum_de/archive/t-29962.html
"Xoana (Schoana) gallego" (http://hfsclub.de/cgi-bin/HFSDB/forum.cgi?showid=13731&fid=7
In many languages, the forms of Joanna have this SH sound: Scottish, Welsh, Irish... And, in English, the Anglicizations of these forms often has a SH: Shana, Shania, Shauna, etc.
2) Or it can be, simply, a direct variation of Joanna based in the voicelessness of a voiced phoneme, because the SH sound in English is the voiceless phoneme correspondent to the J sound in English (which is voiced), and the voicelessness phenomenon of voiced phonemes —as mispronunciation or not, it depends of the case— is usual in languages which have pairs of phonemes voiced and voiceless ([z] and [s], e.g.). In Catalan, for example, Joana (with the voiced sound) is sometimes mispronounced as Xoana (in the social dialect called "xava", basically).