I already know that it's hardly used anymore, but for sentences such as 嬉しゅうございます, is this humble? That is to say, is it inappropriate to use the form for someone of higher social standing (provided they have told you)?
Also, would the past tense be 嬉しゅうございました?
This is called 「ウ[音便]{おんびん}」 and it is one type of the 「音便 (euphonic sound changes)」 that took place around Heian period (794 - 1185).
「ウ音便」, in the simplest terms possible, is the dropping of the "k" consonant from the [連用形]{れんようけい} (continuative form) of i-adjectives.
The 連用形 of 「うれしい」 is 「うれしく」.
Drop the "k" from 「うれしく」 and you have 「うれしう」.
To make 「うれしう」 even easier to say, you will have 「うれしゅう」.
「うれしゅうございます」 is a very polite form, and not a humble form per se as you suspected. It can be used by anyone when they want to sound polite. The past tense is just as you formed it.
Lastly, despite a popular belief among J-learners, this sentence pattern is still heavily used in many parts of Western Japan. Even in Tokyo, the older and more refined speakers still use 「～～しゅうございます」,「～～しゅう[存]{ぞん}じます」, etc.
