Very rough notes to show the type of sytnax that needs to be supported to support user-defined replacement keys (with configurable joinMethod, case-sensitivity and percent-encoding) for the mailto: "open with other application" format string.

http://mail.google.com/mail/?compose=1&view=cm&fs=1&to={"hname" : "to", "joinMethod" : "address", "ignoreCase" : "true", "percent-encoded" : "true"}&su={"hname" : "subject", "joinMethod" : "default", "ignoreCase" : "true", "percent-encoded" : "true"}&body={"hname" : "body", "joinMethod" : "multi-line", "ignoreCase" : "true", "percent-encoded" : "true"}&cc={"hname" : "cc", "joinMethod" : "address", "ignoreCase" : "true", "percent-encoded" : "true"}&bcc={"hname" : "bcc", "joinMethod" : "address", "ignoreCase" : "true", "percent-encoded" : "true"}

{"uri" : "prefix"} is a replacement key object for the full mailto URI including "mailto:"
{"uri" : "noprefix"}
{ starts a replacement key object
} ends the replacement key object
}} = }
{{ = {
{"hname" : "notfound"} will be replaced with "".
User-entered syntax must be perfect and no unsupported variables can be specified. If so, an error should be indicated.

(hnames in the mailto URI are percent-decoded first before comparing)

" outside a replacement object is used for quoting the command line argument (which also then escapes \ and " (where needed) in the value returned by the replacement key object) so it doesn't end the quoted argument too early on the command line). User-entered, non-replacement-object text inside the quotes must be properly escaped by the user or an error will be emitted.

Here's a quoting example:

program:
app.exe
open with other application:
"profile-dir=c:\document and settings\user\desktop\\" "quoted=\"test\"" "-subject={"hname" : "subject", "joinMethod" : "default", "ignoreCase" : "true", "percent-encoded" : "false"}"
uri:
mailto:?subject=%5C

Would execute:

"app.exe" "profile-dir=c:\documents and settings\user\desktop\\" "quoted=\"test\"" "subject=\\"

and the app would get:

profile-dir=c:\documents and settings\user\desktop\
quoted="test"
subject=\

Join methods (searching is from left to right):

address: Join all non-empty instances of the hvalue with "%2C%20" (or, ", " if percent-encoded is false.)

default: Use only the last instance of the hvalue (even if the hvalue is empty)

multi-line: Join the first non-empty instance of the hvalue and all instances after that (even if they're empty) with %0D%0A (or \r\n if percent-encoded is false (or platform-depending newline if necessary))

ignoreCase:

if true and {"hname" : "body"}, body=line1&BoDy=line2 would emit "line1%0D%0Aline2". If false, it would only emit "line1".

percent-encoded:

If true, subject=1%202 would emit "1%202". If false, it would emit "1 2".