No you can't. it's a fallacy. Unless you are prepared to shim the neck to body joint that is.
Not to get too far off topic here...
Since you built this uke, I will not attempt to argue with you. That being said, on most banjos you can adjust the neck angle slightly via the coordinator rod. I would assume that it would be the same with a banjo uke (It definitely is on the vintage Gibson banjo mandolins that I have owned).
When it comes to banjos (in general), it is not a fallacy. Even the two Ome banjos that I own, that have what look like dowel sticks, have spacers on one end that can be added or removed in order to make slight neck adjustments.
The Romero that I used to have used a system that was similar to early Gibson banjos. Here's a description from their website:
COORDINATOR ROD
We're proud to be using a coordinator rod & neck attachment system designed in our workshop here at Romero Banjos. We created this original design here in the banjo workshop early on, and believe it is the best way to attach a banjo neck. OME Banjos and Vance Banjos both liked it so much that they have also adapted our design on their instruments.
Romero banjo necks are attached using a two lagbolt technique adapted from early Gibson banjos, in use since the 1920s. However, rather than having two metal coordinator rods - one attached to each lagbolt - we continue to use two lagbolts to attach the neck and rim but have developed a unique single coordinator rod using machined brass and wood.
Combining the functionality and strength of a metal coordinator rod with the beauty of the wooden dowel stick common to turn-of-the-century open back banjos, our proprietary attachment system results in excellent tone, long-term stability, and ease and adaptability for future adjustments."
Here's a video on adjusting a coordinator rod from another major banjo manufacturer:
Here's Frank Ford's take on coordinator rods:
http://www.frets.com/FretsPages/Musician/Banjo/CoordRods/coordrod1.html
Excerpt from the book "How to Set Up the Best Sounding Banjo" by Roger Siminoff:
Here's Jake Wildwood's blog page in which he writes about making a coordinator rod adjustment on a 1920's Gibson UB3 banjo ukulele:
https://jakewildwood.blogspot.com/2015/07/1920s-gibson-ub-3-banjo-ukulele.html
The strings should not be hitting the head of the uke. My intent was to give the seller (or buyer) some ideas on how to remedy this issue.
Since the banjo ukulele is of your creation, maybe you could offer some insight on how best to remedy this?
All the best,
Ryan
edit: I added three more references having to do with coordinator rods.