Update Dec 2010: it looks like FullTextRSS is down. I’ll leave this up, but be forewarned: it probably won’t work. FiveFilters and WizardRSS provide similar services, so you may want to look there.

Here’s a script that attempts to subscribe to a full-text feed of the current subscription in NetNewsWire. It does this using EchoDittoLabs’ excellent FullTextRSS service.

To use, select a headline or subscription title in NetNewsWire and run the script. The full-text feed will appear in your top-level items.

Thanks to harvey.nu for the URL encoding routine. The script worked without the routine but it seemed safer to include it.

[Updated 2/22 with change suggested by Pascal.]

(*
Fulltextrss.scpt v 0.1b

Attempts to subscribe to the current subscription via
EchoDittoLabs’ FullTextRSS service (echodittolabs.org/fulltextrss)

Contains no error checking; use at your own risk

Dan Byler dbyler@gmail.com
*)

property fulltextpre : "http://labs.echoditto.com/projects/fulltextrss/?url="

tell application "NetNewsWire"
        if exists selectedHeadline then
                set this_headline to selectedHeadline
                set stdfeed to RSS URL of subscription of selectedHeadline
        else if exists selectedSubscription then
                set stdfeed to RSS URL of selectedSubscription
        end if
        try
                set theTextEnc to my urlencode(stdfeed)
                set fulltextfeed to fulltextpre & theTextEnc
                set theresult to subscribe to fulltextfeed
        on error
                display dialog "Oops—something went wrong."
                return
        end try
end tell

– urlencode routine taken from http://harvey.nu/applescript_url_encode_routine.html
on urlencode(stdfeed)
        set theTextEnc to ""
        repeat with eachChar in characters of stdfeed
                set useChar to eachChar
                set eachCharNum to ASCII number of eachChar
                if eachCharNum = 32 then
                        set useChar to "+"
                else if (eachCharNum ≠ 42) and (eachCharNum ≠ 95) and (eachCharNum < 45 or eachCharNum > 46) and (eachCharNum < 48 or eachCharNum > 57) and (eachCharNum < 65 or eachCharNum > 90) and (eachCharNum < 97 or eachCharNum > 122) then
                        set firstDig to round (eachCharNum / 16) rounding down
                        set secondDig to eachCharNum mod 16
                        if firstDig > 9 then
                                set aNum to firstDig + 55
                                set firstDig to ASCII character aNum
                        end if
                        if secondDig > 9 then
                                set aNum to secondDig + 55
                                set secondDig to ASCII character aNum
                        end if
                        set numHex to ("%" & (firstDig as string) & (secondDig as string)) as string
                        set useChar to numHex
                end if
                set theTextEnc to theTextEnc & useChar as string
        end repeat
        return theTextEnc
end urlencode

Download a copy here.