It is a good idea, just realize you're not talking about casting with subtlety.
Casting with subtlety as a mechanic increases the CTN by 6 and the speed by 2. It automatically requires a Stealth (Primary) roll vs. either Passive Perception if not actively being observed or opposed check against Perception (Insight). If successful, no one notices you casting a spell or depending on the spell may not be able to tell the spell's origin. Some spells will get situational benefits such as casting a darkness spell when already in shadows or a penalty - casting a gladius of light in the darkness. Some spells you can't completely hide the effect of, even if you can conceal where it originated.
What you're talking about doing is bluffing about the Source of the spell. There are no mechanical adjustments so casting the spell using this approach is as easy as it would be normally. Discern Residue can determine the Tradition the spell is from, though not the source. If you call out "May Nier's flames burn you to cinders!" as long as it's on the other list you can potentially get away with it. You still draw attention to yourself, potentially more than you would have otherwise for having cried out, but may be able to convince folks that things are as you say. However, confronted with someone who is capable of casting the spells in the Source you claim may prove a lot harder to persuade. The cants of each church are specific, so a priest of Cadic can probably tell listening to you or watching you if you're invoking Cadic's power in the way He intended. If not, you may be treated as a heretic for attempting to masquerade as a member of the Church when you're not.
Far easier with the unwashed masses than the enlightened. 
From personal experience and mapping out progress, casting with subtlety costs you basically 1.5 Tiers of casting prowess at a slower speed. With the Adaptation: Rapid Spell, you can effectively turn a +7 CTN into a Speed +1, Strain +2 spell modifier and not be noticed. Playing a caster as intended by the way the world works, can be really challenging. I find it quite fun to figure out ways to try and do that effectively - such as you've proposed - to help make it worthwhile to offset the loss of effectiveness.
With a sweep of his hat,
Paul
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