I was in a big hurry to try it out today, so the end result could've been prettier than my examples shown here. (Handwritten, crooked labels...) Anyway, the most important thing is that my kids loved them and played with them for almost an hour. My six year old had fun making simple rhymes, and my four year old thought it was hilarious to make towers and have us read the nonsensical sentences he created.
A few notes...
I used blank adhesive address labels, which I cut to size. (Obviously roughly.) You could type or stamp these, or write right on the Lego with Sharpie. I was nervous that my idea wouldn't work, and I didn't want to ruin a bunch of Legos, so I went the label route.
I consulted these directions for making magnetic poetry. It's the same concept, which made it easier to choose words without having to think too hard.
Do make sure to put pairs of rhyming words on different bricks, since kids are likely to want to use them both in one poem.
I made a few phrase bricks, just in case they got stuck. "There once was a..." "who ate a..."
Let me know if you try this. I would love to see your cute creations!