Hi,
Add a miniature drop-down selectbox (just like the ones for skills) somewhere at the screen. Clicking on it opens a small panel (just like the ones for skills) showing all your miniatures, selecting one drops the miniature into the world – selecting the empty slot disables your miniature.
At first this idea sound silly to me, until a mate mentioned that he loves miniatures and that he would buy more of them when such a function would exist, as it would not bloat up his inventory-space with miniatures. So this small idea might sound silly for us, but might be a money-giver for ArenaNet.