I've found a way to manually un-grey the "Create a Windows 7 install disk" option. The greying out is a result of some 'abitrary' restrictions Apple placed on the Boot Camp Assistant. I'll describe step by step the process I went through to bypass the restriction:
1. Let's inspect where the restrictions are originating from. Open up Finder and navigate to /Applications/Utilities, right-click on Boot Camp Assistant. Click on "Show Package Contents". Proceed into the folder "Contents" where we have two .plist files. The one concerning us is "Info.plist". Use Quicklook to preview this .plist and, scrolling to the very bottom, notice the key entitled "USBBootSupportedModels". This is the root of the issue. On my machine, it contained 6 entries by default, none of them corresponding to my mid-2009 MBP.
2. These entries correspond to model identifiers of Macs. While I'm not entirely sure, it's fairly intuitive to deduce that MB is MacBook, MBP -> MacBook Pro, IM -> iMac, MP -> Mac Pro, MM -> Mac Mini, and MBA -> MacBook Air. We're going to go behind Boot Camp Assistant's back and insert another entry into this list corresponding to our machine!
3. With your mouse navigate to the Apple in the Menu Bar, click it, and hover the mouse over the first option, "About This Mac". Holding the option key changes the menu item into "System Information...". Click this item, bringing us into System Information. On the left panel you should be at the first top-level item, "Hardware". In the right panel the Model Identifier is visible. For my mid-2009 MBP it is MacBookPro5,3.
4. We need to insert this information into the .plist file from before; however, the .plist is protected. I had some trouble getting the file to be writeable, but this is the way I did it: I right-clicked on Info.plist and selected "Get Info". I clicked the lock in the lower righthand corner, and input my password to be able to change the permissions of the file. I changed all three permissions to "read and write". Back in Finder, I went up a folder, command-up arrow, and changed the permissions of the folder containing "Info.plist" as well. Only then was I able to freely modify Info.plist in Xcode, my default .plist handler.
5. Create a new entry like the others containing the model identifier information in the appropriate format. If you have a 15" mid-2009 MBP whose model identifier was MacBookPro5,3, the entry should be MBP53. If you have a 13" MacBook Air the entry will be MBA32. Save Info.plist and revert to the previous (non-writeable) permissions of anything you have made writeable.
6. Open up Boot Camp Assistant and the option should be selectable, as it 'should' be. Congratulations, you bypassed this annoying and arbitrary restriction!
NOTE: I've personally not installed Windows 7 using this method. I cannot comment on anything past this point.
I hope this method worked for you all 🙂