Well here is a small trick to get that functionality:
Write a cusom event handler on ItemCheckingIn which validates if the comments are entered; and if not - Cancels the check in with a error message shown to the end user. In the following code snippett an additional check is made for length of characters entered which can ofcourse be removed or changed to a different length. Would be ideal if that length is read from another List which could be called as Config list for a project hence if it was 20 chars - change the config list and we are done.
public override void ItemCheckingIn(SPItemEventProperties properties)
string checkInComment = properties.AfterProperties["vti_sourcecontrolcheckincomment"] as string;
//check for comments being blank or length less than some number of chars.
if (string.IsNullOrEmpty(checkInComment) || checkInComment.Trim().Length < 5)
properties.ErrorMessage = "The Version Check in comments should be atleast 5 characters long.";
properties.Cancel = true;
The key above is the property "vti_sourcecontrolcheckincomment" which contains the comments entered by user. Just to re-iterate ChekingIn event is raised before the document is checked in.