Aleksandar Stamboliyski was a bulgarian politician after the WWI. Actually when Austo-Hungary attacked Serbia he made a speech in the bulgarian parliament: "When our slavic brothers are in danger I'm not bulgarian or serbian, I'm jugoslavjanin", so yes  he is the iedalist of Jugoslav state. Also Georgi Dimitrov had such an idea.
Actually "Dobrodetelna druzina" was not the only with such idea of balkan/jugoslav state. Also this idea was promoted by "Odesko bulgarsko nastoyatelstvo", bulgarians from Tzarigrad and "Moskovski slavjanski komitet". The bulgarian ТБЦК ( a revolutionary organization) first made the suggestion of making a Jugoslav state.