Yes the bar weights can be removed. And no they are not welded in to any significant degree.
That said... Depending on how much room you need, you may be able to simply drive them in an inch or so with a hammer and a punch... After reading the below, you will understand why that may be possible...
OR... You can do what I did and remove these weights...
I've done two bar sets, and have found it's pretty easy, provided you take care with the alignment while drilling and tapping.
Just finished doing this, as part of another project which I will be posting about as soon as I finish.
Here is how I remove the weights.
Note: that while some of these pictures were taken with a junkyard bar I bought for practice and to see if this could be easily done, doing so on the bike is no biggie, as long as you are careful with the alignment of the drill, and take care with the tap, so that you do not break it off in the weight.
First center punch the weight to get the drill centered.
Next using a pilot size drill (I used 1/8") carefully align the drill and drill a hole approx 1.5-2" deep into the weight, followed by a 5/16" drill, drill the weight to the same depth of the pilot hole.
Then drill using a 3/8" drill, to approx 3/4-1" deep... This is for ensuring alignment of the tap you will use in the next step.
Then using a 3/8-16 tap, cut approximately 5-6 threads (5/16-3/8" deep) into the weight. Be careful with this step and use proper cutting oils, so you do not break the tap off in the hole.
Finally I used a small slide hammer consisting of an approximate 12" long grade 5 or better 3/8-16 bolt and an approx 10-12 oz weight to remove the weights. Takes about 4-5 sharp whacks, and the weight will be slide right out, as the only weld is a small (less than 1/4" dia) resistance weld, as you can see in the pictures...