Is there any add on or hack that would allow tagging Inbox by Gmail reminders?
Inbox by Gmail doesn't support add-ons at this time.
Regarding reminders hacks, you could set reminders in email messages and Google Keep notes both of them allow to be labeled. I.E. send an email to yourself, assign to it the labels that you want then set a reminder. In Google Keep just create a note, add labels to it an the desired reminder.
From the reference
View Google Keep reminders in Inbox
You can view reminders you created in Google Keep in Inbox by Gmail.
The title of the reminder is based on the title and body of the note in Google Keep. To view or edit the note, click the Open in Google Keep link under the reminder.
Learn more about reminders in Google Keep.
Reminders - Inbox by Gmail Help