How to pad messages with large amount of padding
As you note, PKCS7 padding isn't designed to do exactly what you want; it's really designed to allow you to pad up to the next multiple of the block size, that is, to the next multiple of 8 or 16.  That it does rather well; however, it's not designed to do what you want with it.

I would note that for block ciphers, as long as you also include a good Message Authentication Code or some other way of ensuring integrity, that the actual padding method isn't critical to security; the AES encryption itself will ensure that there is no information leakage other than message length, and the MAC will ensure that, if the attacker tries to play games with modifying the message, well, that'll always result in a MAC failure, and so that attacker won't learn anything).  This is true even if you try to design a padding scheme specifically to leak information (assuming, of course, that your padding method doesn't use the key).

Hence, the only real constraints on your padding scheme is:


Not to leak any information do to the message length (that's the one thing AES does not disguise)
Be able to remove the padding to obtain the original message without ambiguity


As long as you follow those two constraints, you're golden.  One obvious way to meet both goals is to pad the message out to a fixed length; and then add a 2 (or 4) byte 'original message length' at either the beginning or the end of the message.

I'm assuming that you aren't concerned about interoperability with existing systems; if you are, then you'll need to live within whatever system they are using.

And, the above answer "the padding method is not critical" applies only to block ciphers along with some MAC); when you consider other cryptographical primitives (say, RSA), the padding method does become important in those cases.