If I write simple text files to a thumb drive using a Windows PC, can they be read later on a Mac?
If so, can they also be locked and unlocked with a software program (not encrypted) if I forward the password?
Problems with special characters might occur when the application you use for writing chooses a character encoding that isn't recognized by the application that the Mac uses to open the file. But generally, using Notepad on Windows and TextEdit on OS X, you should be fine.
Depends on what you really need to do.
There is no cross-platform lock/unlock tool I am aware of (like there is Folder Lock for Windows). It wouldn't really make sense too. Think about it: Such a lock software would have to be installed on the client systems in order to monitor access of the file in question. It could easily be disabled and wouldn't be of great use after all.
The easiest way to make sure that a file isn't readable and editable by an unauthorized person/program would be to encrypt it anyway. From the end-user's point of view it's more or less the same thing. Consider taking a look at how to create a TrueCrypt container.
If you want people to have read-only access to the file, but require them to enter a password only to edit it, you're probably looking for another way of exchanging information too.
Make sure the thumb drive is formatted to a file system both OSs can read. Both can read FAT12/16/32/exFAT and NTFS, but Mac OS X cannot natively write to NTFS. If your preferred file locking/encrypting tool requires the ability to write to the drive, then don't use NTFS.