This is an interesting idea, maybe a good small summer project. The only open source clipboard manager for Windows that's worth mentioning is Ditto. It has a lot of good features, the clipboard sync is a pretty amazing idea, although it doesn't support password protection of the buffer - but hey, it's open source as you requested, if you have the knowledge to tweak the code then go for it!
It stores data in a sqlite database though, which you could password protect with TrueCrypt. Then you'd only need to authenticate once when you log in.
Another possibility is to use AutoHotkey. If the people using this computer aren't fairly tech savvy, you could compile your script with AHK2EXE and that should be fine. Although if they do know their way around things they may know about disassembling and programs like ollydbg which will show the password strings in plain text.
An example:
!p::
InputBox, password, Enter Password,, hide
if (password == "secret")
{
Send My Secret Text.
}
else
{
MsgBox Access Denied.
}
return
or use a regular clipboard manager for standard clipboard tasks, and use AHK with a master password to store all other passwords:
passes =
(
SuperUser:secret
StackOverflow:secret2
Meta:secret3
Gmail:secret4
)
!p::
InputBox, password, Enter Master Password,, hide
if (password == "secret")
{
MsgBox %passes%
}
else
{
MsgBox Access Denied.
}
return


As previously stated, this is not a secure method if savvy users will be on this system, however it's fine to protect against the average user if compiled with ahk2exe. No real point since you could use one of many password managers, but it is an easy way to have them available in a few key presses. I'd recommend the Ditto and TrueCrypt method though.