Hmm, Dreamer used to be our "key manager" but unfortunately Dreamer is no longer around. By default, the shortcuts for XnView Classic and XnView MP should be the same, so the FAQ
"All Current keyboard Hotkeys & Shortcuts" helps to find a short cut for the draw tool. Though, I'm not sure whether the list of keyboard shortcuts is up-to-date.
According to the list the following Shift+<key> combinations are still free in view mode:
Shift + B, D, F, G, I, J, K, N, O, P, Q, U, W, X, Z
And the following single keys are supposed to be free:
B, E, G, H, J, K, L, N, O, P, Q, R, S, U, V, W, X, Y, Z
Shift+D would be a good shortcut, I think. Other suggestions?