FCC / CE requirements for adapter pcb?
If you want to sell the thing in Europe, you need to affix CE. For CE, the intended use determines what directives you need to comply to.
This is probably limited to:

Electromagnetic compatibility
Restriction of Hazardous Substances in Electrical and Electronic Equipment

You can use harmonised standards to make very plausible your product is satisfying the EMC directive, and while it is one of the best ways to do it, you don't have to.
I think when you connect the shields of the USB such that it forms a cage of Faraday, you can make use of article 2 (d) of the EMC directive

equipment the inherent nature of the physical characteristics of which is such that:
(i) it is incapable of generating or contributing to electromagnetic emissions which exceed a level allowing radio and telecommunication equipment and other equipment to operate as intended; and
(ii) it operates without unacceptable degradation in the presence of the electromagnetic disturbance normally consequent upon its intended use;

The generating of EM emmisionis obviously not present. The contribution and susceptibility to EM emmision is made plausible by the Faraday cage.