Trim fat from lamb. Place lamb in an electric slow cooker, and sprinkle with oregano, rind, and 1/4 teaspoon salt. Pour in juice; top lamb with spinach. Cover with lid; cook on high heat for 1 hour. Reduce heat setting to low; cook 7 hours. Remove spinach and lamb from slow cooker, reserving cooking liquid; cool. Remove meat from bones; discard bones, fat, and gristle. Chop meat.
Place a zip-top plastic bag inside a 2-cup glass measure. Pour drippings into bag, and let stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner of bag. Drain drippings into a large bowl, stopping before fat layer reaches opening; discard fat.
Add 1/4 teaspoon salt, lamb, spinach, orzo, and feta to strained drippings. Garnish with oregano sprigs and lemon slices, if desired.