Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a deeper, technical meaning?
Non-portablility isn't about sales... it's a technical term.
Non-portable IP addresses belong to a certain organization, and an end-user of the non-portable IP address is not permitted to discouraged from ...
$ whois -a "o ! > HT-136"
As found by doing man whois and whois -a ?
Use the American Registry for Internet Numbers (ARIN) database.
It contains network numbers used in those parts of the world cov-
ered neither by APNIC, AfriNIC, LACNIC, nor by RIPE.
As Ricky Beam told in comment, the ARIN database is not available in a downloadable (archive) format.
But you can query it directly using REST.
As stated in the ARIN WHOIS-RWS web page
ARIN’s Whois RESTful Web Service (Whois-RWS) is the new directory
service for accessing registration data contained within ARIN’s
How do I use ...
I guess I should start by saying that IP prefixes are not technically bought. When an IP prefix is allocated to a provider from an RIR like ARIN, there is no transfer of ownership. The IP range is always allocated to the RIR, and the allocation to the provider is only valid so long as the original terms of the allocation are upheld.
As for the routing ...
As waza-ari pointed out, AS Prepend will do the trick. As to whether it's what other ISPs do, I cannot answer that question unfortunately.
I labbed up your AS prepend query using the topology below:
AS Prepend Example:
R1 is advertising 192.168.1.0/24 through BGP.
It is prepending 123, 123, 123 to the route.
The below output was taken from R2:
Your Org can create a RADB entry for your customer prefix (since LOA is given, that should be mentioned in the "remarks" column of RADB) under your maintainer object. When your ISPs query RADB now, they will find a match of the prefix against your ASN and allow the prefix.
As a potential alternative to that, if you have some kind of connection to AWS from where you’re currently using your ARIN-assigned block, such as via a site-to-site tunnel or Direct Connect circuit, you could use one of your available ARIN IP addresses and NAT it to one of the AWS hosts’ private IP addresses rather than having a public IP address assigned ...