Concerning booting differently depending on the presence of a USB key :
What I would do is have tow grubs. One on the PC, which boots immediately to windows, without any hint that anther system exists. A second grub would be installed on the USB key. This one would give you the actual boot options, and have the key to decrypt the partition. Then, you can specify in your bios/uefi to boot from usb by default. You will then obtain your desired behaviour. Keep in mind though, enabling USB boot can be dangerous. You have to know if this is a concern for you. (To reduce this risk, look into using secure boot. Don't quote me on this, but I believe some UEFIs let you specify your own keys for the OSs to allow boot for.)
Concerning "hiding" the suse partition :
As has been already said, the Suse partition cannot be invisible (except maybe if you play with the HDDs firmware?). However, depending on the filesystem you chose for it, it is very possible that it would not be visible from within windows, to a user who is not looking for it. (Windows alone cannot read ext4 for example, and so any partition formatted in ext4 would not appear in "My Computer". It can still be found with the appropriate tool though.)
Whether it's "reasonable" to do :
This is really subjective. This technique would add a layer of security, which, by principle, is always a good thing. But it really is not a robust protection scheme. "Hiding" the partition will make absolutely no difference in case your attacker is skilled and determined. Off the top of my head, The only case where this would be actually useful is for airport security checks. Sometimes security officers ask you to boot your PC, and can become very suspicious if you have a "strange" OS running. Then you can just show them the Win7 boot they want to see.
Encrypting your important partition and keeping the key on an external device, though, is definitely a go !