You should be able to get furniture to raise motives by editing the ITUN (0x03B33DDF) for the interaction you wish to change the motives. I was able to edit the Take Shower/Take Bath ITUN files to increase fun while in the shower by doing the following.
I'd imagine doing -your number on the actual= part would reduce the motive. You'd just have to change the Change type= to match what you want it to do.
Also you might not want to put 1000 as your advertised value. I just realized that this is a bad idea myself since no controlled Sims now use the shower obsessively to the point that I watched two of them play musical showers by getting out of one shower and into the other.