The context is indeed relevant. If you want the memorial to be, in a sense, a list of people, the nominative is appropriate. If you instead are dedicating the memorial to the listed people (as is often done), then it is the dative which is appropriate.
Thus, in either case, I would propose something like
et omnes quos amavi (nominative, masculine)
et omnibus quos amavi (dative)
(both meaning "and [to] all whom I loved"; the second one having the "to")
Notice that for the former example, I have selected the masculine (qui) for the general gender of the group (as it is often done in some languages, e.g. Spanish), but you can also select the feminine, quae (an issue Tom's answer doesn't need to deal with). The second option above does not have such gender choice.
Additionally, one can say "and [to] all those whom I loved", noticing that "those", like qui/quae, is no gender neutral in the nominative. Thus, you would need to pick one: illi or illae for the masculine and feminine, respectively. The dative is the same, illis. Thus, you can have:
et omnes illi quos amavi (nominative, masculine)
et omnibus illis quos amavi (dative)