As a variant of what Stefan has suggested:

Perhaps a simple comment is set up as the pre-loaded reply - perhaps including date and time.  Maybe include the author's name in the comment, but all that is available anyway (see documentation for that).  This bit isn't vital.

Then each user simply has to *reply* to the message concerned, and you then get the list of who has read it, available in threaded view of the comment and replies.

Of course this works best with small numbers of users, and where threads are never very long.  Also, it means that you can lock the original message (prevent it being edited) which may be a particular concern depending on what you are using elog for.

You might also want to define an attribute that a reader can tick to say that they have taken action if it is important that someone has to take further action (as opposed to just reading) - and maybe that changes the colour of the background or the icon in front of the topic.  Then there would be a record as to who has done the necessary, and later readers will know the matter is for information rather than needing attention.   For example,. who kicked the power supply to the ion source on Monday morning to "wake it up" (this is true, but pre-elog, so was a paper record).  John kicked it last Monday, and the power supply respects John, may be a statistic one could find from such information recorded as suggested here.

You may want a second attribute people have to explicity select so as to show they understand what they've read, rather than just reply/submit without having read... e.g. they have to select a digit of the ticket number, or some auto-generated number (or letter, I'm not characterist) that is within the original comment.

We all know people who click on things they've never read, and come on, we've all done it at times...

David.