Portugal is quite at ease with the history of its colonial past and its role in the Atlantic slave trade, there is no denial or attempt to hide the historical facts and the slave trade and the role of slavery in the former colonies (specially Brazil) is well discussed during obligatory schooling.
All this to say I can't really understand why Lisbon should acknowledge its slaves to such great length as this post asks to, because in fact Lisbon had very few ones actually living there (it was not well seen in Catholic Europe to own slaves) and was mostly a point of passage (but then again, so were many Atlantic islands).
If you want to check the historical facts: https://en.wikipedia.org/wiki/Atlantic_slave_trade