It looks like most people do not understand your question ... You reminded me, I used to do something similar with Windows 2K and openSUSE a while ago, chain-loading with the Windows boot-manager upfront. I think what you're aiming for is just editing a single configuration file within the Windows system partition (or "EFI partition").
In the old days, you needed to edit a file called boot.ini, now it is some sort of a registry file called "BCD file." There is a reference by Microsoft:
http://technet.microsoft.com/en-us/library/cc721886%28v=ws.10%29.aspx or http://technet.microsoft.com/en-us/library/cc731245.aspx etc
Microsoft recommends to edit this file with a Windows command line tool: "bcdedit". But all you need to do is to change the timeout, I guess. So maybe you can locate the registry file (as described my Microsoft) and edit it with a text or hex editor from Ubuntu (*). The timeout is a number, witch can be found only once in this file.
(*) I cant find a sample file on the internet and I have not touched Windows since version 2k. If you find it, you may paste it here or at some convenient place (and place a link here).
Be careful with writing into NTFS partitions ... NTFS-3G is known to do a good job.
(Your best option is really to get a Windows 7 DVD somewhere. You do have a licence, so you can maybe just order another copy. There is some documentation on using bcdedit with wine, but most of it can be found in the bug-tracker of the wine-project, so I would not even touch this option. Besides, I could not find Linux-live-systems, which include utilities for editing this stuff. There is a program called EasyPCD, which is going to be released as a life system for some 25 US bugs, but right now it is not really there. The only real alternative appears to be the successor of the ERD commander, which is only available to enterprise costumers of Microsoft. But it does in fact include something like the bcdeditor. Last but not least, BartPE is an option, theoretically. But again you need a Windows DVD, before you can [legally] create your own PartPE CD/DVD.)