I've never heard of a "rokay ki rasam" in Pakistani culture - however I do have hindu/sikh friends and they have a "roka ceremony". Here's a blurb on what the roka ceremony is about:
Roka Ceremony
Roka ceremony is performed on a mutually convenient date backed by 'shubh muhurat'. This is a simple function, which is attended by family members and close friends. There is normally an exchange of gifts and cash. After this roka ceremony, the girl and the boy are considered engaged to each other and are also allowed to court. The wedding date is fixed after roka. (Source: Seasonsindia.com)
Roka Ceremony
Roka ceremony is basically carried out to secure the commitment of both the parties. It is a small function involving the family members and close relatives of both the prospective bride and groom. The families involved exchange gifts, sweets, fruits and dry fruits. (Source: Wedding.Iloveindia.com)
So to answer your question, since the rasam is sort of like an engagement, without necessarily exchanging rings and less formal - I'd wear a semi-formal outfit - nothing too OTT.