<p>When issuing an HTTP DELETE request, the request URI should completely identify the resource to delete. However, is it allowable to add extra meta-data as part of the entity body of the request?</p>

The spec
does not explicitly forbid or discourage it, so I would tend to say it is allowed.
Microsoft sees it the same way (I can hear murmuring in the audience), they state in the MSDN article about the
DELETE Method of ADO.NET Data Services Framework
:
If a DELETE request includes an entity body, the body is ignored [...]
Additionally here is what
RFC2616
(HTTP 1.1) has to say in regard to requests:
an
entity-body
is only present when a
message-body
is present (section 7.2)
the presence of a
message-body
is signaled by the inclusion of a
```
Content-Length
```
or
```
Transfer-Encoding
```
header (section 4.3)
a
message-body
must not be included when the specification of the request method does not allow sending an
entity-body
(section 4.3)
an
entity-body
is explicitly forbidden in TRACE requests only, all other request types are unrestricted (section 9, and 9.8 specifically)
For responses, this has been defined:
whether a
message-body
is included depends on both request method
and
response status (section 4.3)
a
message-body
is explicitly forbidden in responses to HEAD requests (section 9, and 9.4 specifically)
a
message-body
is explicitly forbidden in 1xx (informational), 204 (no content), and 304 (not modified) responses  (section 4.3)
all other responses include a message-body, though it may be of zero length  (section 4.3)