I am, other than dumbell press Im only doing body weight exercises. Just looking for a core workout that will replace one of my standard days.
Was it you I linked to that book Building the Gymnastic Body? It's a good one for all this stuff.
Human flag specifically....
Weighted pull-ups. These are extremely helpful. Many think flags are all about core strength, but the reality is that there is a massive shoulder and arm component to the exercise. The upper arm has to pull with quite a large amount of force while the lower arm needs to lock out. Not just the elbow, but the shoulder as well. So one needs very good upper arm and shoulder strength. So that ties into the next exercise. 1.5X bodyweight is a pretty good standard to try to get to for Human flag strength.
Handstand holds. Try not to arch the back too much here. Straight to slightly hollow is the new standard. Even against a wall works well. For handstand push-ups to make it better for the flag make sure to lock out the elbows straight at the top of each rep. Eventually practice with a wider hand placement, eventually double shoulder width, elbows locked out, and lean the legs from one side to the other.
If you have access to a gym... using one of those back extension things but on your side, while holding a weight straight-arm overhead and doing side-bends can help to prep the body.
Another somewhat helpful exercise is the hanging windshield wiper. Hang from chin-up bar, keep arms straight throughout, bring toes up to bar, and bring them down to the side to parallel with floor and back up on each side. TO add a little something more do full 360 rotations and back around, like a clock, while keeping arms straight.