A few former Boy Scout leaders and some former members of the German youth movement visited the last pre war Jamboree in 1937.
They traveled as tourists to the Netherlands, or had left Germany earlier and were living in France, Belgium or ne Netherlands by 1937.
They did not show up in any German Scout uniform, since the were some Germans around who were suspected to work for the Gestapo.
In the Gestapo files an interview of one German visitor of the Jamboree can be found. There was no German contingent officially invited.
No Germans are mentioned in the official publications of the time. Some publications are in German for Swiss and Austrian visitors.
There is even an official camp-booklet in German. (Besides a Dutch, a French and an English edition). And no, no HJ invited for sure.