The 'Name' attribute of a snapshot is actually a tag. As such, you can change/set it with the
ec2tag command line tool.
ec2tag snap-xxxxxxxx -t Name=SNAP_NAME
As per the help file:
ec2addtag ([ec2-create-tags, ec2tag])
ec2addtag [GENERAL OPTIONS] RESOURCE_ID [RESOURCE_ID ...] --tag KEY[=VALUE] [--tag KEY[=VALUE] ...]
Any command option/parameter may be passed a value of '-' to indicate
that values for that option should be read from stdin.
Adds a set of tags to a set of resources. To update a tag's key or value, you also use CreateTags.
The new values overwrite the old values.
-t, --tag TAG
Tag in the form of key[=value].
ec2tag is the 'shortform' of
ec2addtag - they both do the same thing. They are included as part of the EC2 API tools.