Since the last half of the password is partially determined by the first half, this doesn't add much additional entropy (an attacker only has to guess your phrase, and which options you've selected).
In practice this may help in a security through obscurity way, but now your method is public.
This may work better for me since I've used phrases from out-of-print books and some are latinized phrases in Sinhalese. I prefer to use random passwords that I can add to a master file that's PGP encrypted, but in the absence of that, I can tailor this to a site and add character rotation.
In practice this may help in a security through obscurity way, but now your method is public.