code writing

One of the things which first got me into languages I think, was writing in code as a kid... my friends and I used to make up all sorts of codes for secret communication, and even as I got older I still felt like speaking in a foreign language was some sort of code which allowed me a greater freedom to say anything I wanted. But recently I was trying to come up with a way of writing that would be a more or less private code, utilizing an already existing writing system and a language I already know. Obviously writing any romance language with an Arabic script has a precedent and anything written in a relatively widely used script would not take long to figure out, so I started looking for "mutually exclusive" languages. One of my initial impulses was to try to write Arabic with Hangul script, since there is almost no logical social, political, or economic reason anyone would know both of those languages, but there were just too many sounds that couldn't be represented accurately. My next attempt, which I undertook a bit more seriously, was writing Arabic in Georgian script, because why make up a script when you can use an existing one that looks made up?

Letters	Unicode	Name			IPA
ა	U+10D0	an			/a/
ბ	U+10D1	ban			/b/
გ	U+10D2	gan			/ɡ/
დ	U+10D3	don			/d/
ე	U+10D4	en			/ɛ/
ვ	U+10D5	vin			/v/
ზ	U+10D6	zen			/z/
თ	U+10D7	tan			/t̪ʰ/
ი	U+10D8	in			/ɪ/
კ	U+10D9	k’an			/kʼ/
ლ	U+10DA	las			/l/
მ	U+10DB	man			/m/
ნ	U+10DC	nar			/n/
ო	U+10DD	on			/ɔ/
პ	U+10DE	par			/pʼ/
ჟ	U+10DF	žan			/ʒ/
რ	U+10E0	rae			/r/
ს	U+10E1	san			/s/
ტ	U+10E2	t’ar			/t̪ʼ/
უ	U+10E3	un			/ʊ/
ფ	U+10E4	par			/pʰ/
ქ	U+10E5	kan			/kʰ/
ღ	U+10E6	ɣan			/ɣ/
ყ	U+10E7	q’ar			/qʼ/
შ	U+10E8	šin			/ʃ/
ჩ	U+10E9	čin			/ʧ/
ც	U+10EA	can			/ʦʰ/
ძ	U+10EB	ʒil			/ʣ/
წ	U+10EC	c’il			/ʦʼ/
ჭ	U+10ED	č’ar			/ʧʼ/
ხ	U+10EE	xan			/x/
ჯ	U+10EF	ǯan			/ʤ/
ჰ	U+10F0	hae			/h/

The Georgian alphabet has almost enough parallel sounds to cover all the arabic phonemes:

ا ა

ب ბ

ت თ

ث ტ

ج ჯ

ح ჰ

خ ხ

د დ

ذ ძ

ر რ

ز ზ

س ს

ش შ

ص ს

ض დ

ط თ

ظ მ

ع

غ ღ

ف კ

ق ყ

ك ქ

ل ლ

م მ

ن ნ

و უ

ي ი

ه


as you can see I wasn't quite sure what to do about the 'ayn and the ha, but beyond that was a deeper issue of historical Arabic-Georgian interaction. While most people focus on the translation movement from Greek to Arabic in the 9-12 centuries due to the recent attention by Dimitri Gutas and the like, at roughly the same time Arabic Christian thought was being preserved by Georgian monks:
...from the modern scholarly point of view, one of the most important contributions of Georgian monks in the Judean desert monasteries, particularly in the early Islamic period, was their activity as translators. Numerous texts, originally written in Greek and Arabic, have survived into modern times only because they have been preserved in Georgian translations.
(from Griffith, 1997)
So, that might be one reason why Georgian-Arabic wouldn't be such a great combination for a tough to crack language code, but I imagine most of those Arabic-reading Georgian monks are dead now. Referring to the Linguistic Mystic's manual on cryptorthography I am tempted to throw a little Cyrillic in there, but I think it is too commonly known, and again there are a lot of Arabic sounds that aren't accounted for.
Short of stooping to "extinct" languages like Nubian, or recently invented scripts for languages that have almost never been written, like Zaghawa, I am not out of brilliant ideas. There must be some efficient way of finding the 2 most widely spoken languages with the least number of common speakers... any guesses?