I don't know Check-In/Check-Out feature of Dreamweaver. So I don't really understand what you want, but want to nevertheless suggest something which is maybe an acceptable alternate working method for you.
In UltraEdit there is the configuration setting Open files as read-only by default
at Advanced - Configuration - File Handling - Load
. With this setting enabled, all opened files are by default read-only independent of the read-only file attribute. Read-only files are displayed in UltraEdit since v16.00 with a user customizable different background color than editable files. Also there is a small padlock symbol left the file name in the file tab of a read-only file.
By clicking on Edit - Read Only
you can toggle the read-only mode at any time for the active file. So you can open all files by default read-only and make some of them editable by executing this command when the file to edit has the focus. The command can be also added to a customized toolbar which I have done to easily see if the active file is read-only (not really needed anymore since UE v16.00). And the command can be executed also by a hotkey or chord by assigning a key code you like to command EditReadOnly
in the key mapping configuration dialog. See also topic Setting a background color for single files manually
where I have explained the read-only settings and behavior more detailed.
Perhaps with using the read-only mode feature of UltraEdit you can work completely without the check-in / check-out capability of Dreamweaver.