I think the answer to your question is going to depend on whether your workflow automatically updates the Account's LastActivityDate Asynchronously whenever the Account History object shows a change. Its possible it may be done as a batch operation as opposed to an asynchronous one. I honestly don't know how its typically done, but am trying to take a stab at providing some timely assistance.
If the account is updated asyncronously, you'd need the trigger to be one that fires "on update" of the Account's LastActivityDate. You'd then want to query the Account History for the fields that contain the objects you're looking for the activities which would have the subject lines you're wanting to filter on.
If workflow doesn't update the Account asyncronously, then you'd need to make the trigger fire "on update" of the Account History object, query Account History for the subject lines you're looking for, then update the LastActivityDate in the Account. You'd also want to look at how any batch programs update that field which might affect how your trigger operates, or more accurately, what the trigger accomplishes for your organization.
Hope this helps you get started. :)