@taldcroft - I'll ask a more general question on whether this is the right approach on astropy-dev, but since this certainly is very neat, here some comments more specific to your suggestion (in order, though none essential):

1. For number-of-digits, I'd suggest following the de-facto standard, e.g., %<length.#ofdigitsafterpoint>DD. Not fond at all of using number of additional d's as in the %DD.ddd example (really uncomfortable if one wants >5 digits, which is not that unrealistic). Effectively, of course, this means the format becomes rather like that of int/float (depending on whether #ofdigitsafterpoint is mentioned), with the letter(s) indicating what is being formatted.
2. Related, I'd hide the %+ option in the above (again following standards); e.g., %+5.2DD.
3. Also related, what is the default # of digits? For most, 2 would be right, but degrees can have 3. Perhaps particularly relevant for %HMS and %DMS.
4. Why caps rather than lower case? To avoid confusion?
5. I'm not in favour of any additional unit stuff, as this is possible already with standard python formatting (e.g., "{:10.5f}".format(a.to(u.mas).value))

@mhvk - thanks for the feedback.

1. Something like that occurred to me and was a close second to the DD.ddd idea. I would be fine with your de-facto standard idea. In fact it does sound better now since it also allows solving the fixed vs. non-fixed width issue nicely.
2. I didn't remember the %+5.2 syntax. Even better!
3. Not sure what should be the default, but with this syntax it can be configured quite easily.
4. I started with caps just because I was thinking of HH:MM:SS.sss which I sometimes write for times. But I was pretty close to updating the proposal to use lower case directives when I had to leave the keyboard. So I'm on board with lower case as well if it ends up looking more clear.
5. I see your point here. This might need iteration but is second-order for now.

It might be worth noting that part of my madness is a desire to have the Angle object be fully aware of how to output itself via configurable object properties, not just keywords in a specialized output method. You've seen this idea in Time. I want code like this to work:

>>> t = Table()
>>> a = Angle([1, 2], unit=u.deg)
>>> t['a'] = a
>>> print t
a
deg
---
1
2
>>> t['a'].format = '%dms'
>>> t['a'].precision = 5
>>> t['a'].style = 'colons'  # Maybe "colon-sep" ?
>>> print t
a
deg
---------------
1:00:00.00000s
2:00:00.00000s

>>> t['b'] = t['a'] + 10 * u.deg
>>> print t
a               b
deg             deg
--------------- ----------------
1:00:00.00000s  11:00:00.00000s
2:00:00.00000s  12:00:00.00000s