indenting wrapped lines in XML

I've been struggling to figure out how to modify the indentation
behavior for XML files when attributes are continued among multiple
lines.

Here's a summary of what I'd like to see:

<parent attr1="one" attr2="two
attr3="three"
attr4="four">
<child attr1="one"/>
</parent>

with two-space indenting, I'd like to have a wrapped line (attr3 and
attr4) indent one extra level; in this case, I'd like a total of four
spaces... 2 for the standard next-level indentation, plus 2 for a
"bonus because it's wrapped" indentation.

I *think* this is the same behavior that cinoptions=+2 would give for
C-style files and the like.

Has anybody tackled this or something similar?

Thanks in advance,
Jim