Often we talk about "what if.." one day it was allowed to write klingon using lower case.

Some would like that, and others not.

Out of curiosity though.. if we *did* write in lower case, how would we distinguish between the q and Q ?

You'd have to make a choice. Since q sounds much more like a k than Q does, I'd turn q into k and leave Q alone.

Vaj toddujdaj ngehbej Divi'. Nimbus wej wighos; he yinab!
Reh Divi' duj visuv vineh.

Dopdak kul yichenmoh qobdi' ghu'.
Reh suvrup suvwi''a'.
Hagh kohpu' neh heghtahvis suvwi'pu'.
Hivka' veklargh.

Qo'nosdak paw cha' Divi' bek.
Tlhingan 'avwi' lughom.
Lutlhob, nadevvo' Vas'a'dak majahlah'a'?
Jang 'avwi', lichopbe'chugh ghewmey.

Tlh: Nukneh?
H: Javmah tat chu'wi' vinehbej.
Tlh: Vaghsad deq hinob! Dah yidil!
H: Kay'be'. Titlhap!
Tlh: Maj. Ha'! DaH matlhutlh.

H: Jabwi', Romulus hik vitlhutlh.
Tlh: Jabwi', cha' hivje' tikem! Tera'ngan motlhbe' soh.
H: 'iwlij jachjaj.
Tlh: Reh hivje'lijdak 'iwghargh datu'jaj.

Personally, I find this very appealing.

-- 
SuStel
http://trimboli.name