You can actually cheese this by going to the Unary Operators/* dereference operator setting, and unchecking the setting. These settings are 3 state - on forces a space, off will remove a space (unless another rule takes precedence), and unchecked tells it to leave it as is in the source. So this way, it won't eat the space after "sizeof" (which it should not do anyway), and the pad-parens setting will work as expected. On the bad side, if you have something like "* bar", it's not going to clean up that space after the "*" either.
I'll try to address this in v21 - we should not be eating the space after sizeof, so that's a bug; and you can make a good argument that pad-parens should take precedence over the unary/postfix space settings for the first and last arguments of a function call, but I'll need to see how that one effects our regression tests to see if it does anything unexpected.
Thanks for the report.