We went to DL this past August and had originally planned to sprint to Nemo when the gates opened during one of our days there to avoid the 2-3 hour lines. Then one evening, I got the very idea you mentioned and my brother's family and my family (4 adults and 5 kids) ended up getting in line at about 8:50. The fireworks started at 9:25. You do not have an unobstructed fireworks view by any means from the Nemo line, but you get to see enough of the show to help the line move a little faster. You can also hear all of the audio. If you want a great view of the fireworks, I do not recommend this, but if you want to see 35% of the show and hear all of it, it makes the line move faster.
Also, to your point, the line was only 70 minutes at this time compared to much longer times during the summer rush. The line does move more quickly than I had thought and the projections of wait time from the cast members at the front of the line were dead-on. So, I heartily recommend this strategy - grab the kids some ice cream, and they won't notice it is a 70 minute wait.
One other thing is that part of the queue is immediately next to the lagoon giving you a nice view of the ride.