Bryan's role in anti-imperialism is exaggerated. The Democrats/Populists in 1896 took a pretty hard line for a "free Cuba" in 1896 (much to the dismay of the Clevelandite anti-imperialists). Bryan supported the Spanish American War, and much to the dismay of the Anti-Imperialist League and supported Treaty of Paris ending the war. That treaty, of course, annexed the Phillipines and the vote was on it was razor thin.
It is true that Bryan opposed "imperialism" in the 1900 election but the anti-imperialist leaders (many of whom were goldbugs) never really trusted him, though most reluctntly voted for him or stayed home. By the end of the campaign, he was pushing free silver again and downplaying anti-imperialism.
I am sure that you agree terms like "liberal" and "conservative" (at least in modern terms) had little meaning in 1900. Bryan was certainly not a liberal on social issues, such as religion or states rights and T.R. was not a conservative on economic issues. Cleveland, on the other hand, is best decribed as a classical liberal Jeffersonian. People like Reed were Hamiltonian in their economic views.
Having said this, the leadership of the Anti-Imperialist League (both before and 1900) were made up of Clevelandite classical liberals who were pro-gold standard, pro-free trade, and anti-big government. They included the Edward Atkinson (a free market fan of Bastiat), J. Sterling Morton, another friend of the gold standard and former Secretary of Agricutlure, William Graham Sumner (the Milton Friedman of his day), Moorfield Storey (a rapid friend of the gold standard and civil rights), and many others. The Bryanites had virtually no role in the top leadership of the League.