I'm not sure what the best method for this. Using the example on that page (because I'm too lazy to make a new puzzle or find another example), and with R3 and C3:
Method 1: Shape of the trimino for each cell, like LITSO, using the characters I-rLJ7 (IrrI, 77rX)
Method 2: Like Method 1, but only I for straight and L for bend, not caring about orientation (ILLI, LLLX)
Method 3: Lengths of contiguous white squares, like shading puzzles but applied to borders (121, 21)
Method 4: O for centers of triminoes, X otherwise (XOXO, OXXX)
Or something else? Ideas?
Of course, for Method 1, we can use a different set of letters.