Yes, of course. The WABEO team maintains mail-in voting locations in the database of all bookable polling locations. These can then be booked just like any other polling locations. However, this data is only available a few days before the election and may also be incomplete because municipalities are only required to publish the counting locations for mail-in voting 7 days before the election. This makes it extremely difficult to get this data.
It also takes longer to observe mail-in vote counting. Preparation for counting usually begins early in the afternoon (between 2:00 - 4:00 PM depending on the municipality). The actual counting of ballots then takes place like in a regular polling office from 6:00 PM onwards. So You need to bring more time if You want to observe a mail-in voting location.
Because there are more processes in mail-in vote counting than in ballot box voting, we recommend that all observers in a mail-in district take WABEO's training and get certified.