I'm not sure if it's how I generated my keys (did the standard gpg keygen using 4096), or what, but I was getting this error when I tried to sign:

java.lang.RuntimeException: Error writing pending entry '268'.
    at org.redline_rpm.header.AbstractHeader.writePending(AbstractHeader.java:182)
aused by: java.nio.BufferOverflowException
    at java.nio.HeapByteBuffer.put(HeapByteBuffer.java:189)

With a bit of debugging it appears that the default SIGNATURE_SIZE size of 287 wasn't enough for the apparent 543 length ones in these keys? Bumping the size up to 543 fixes the issue.