Yes, UNC will have issues because the LANDESK agent runs as local system which would not have network rights. You should look into the use of Preferred Servers where credentials get provided to the agent for accessing shares.
A quick fix might be to set the package so that it runs under an account that would have both the rights to install the package on the client as well as access the share you are pointing to.
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Don't forget that LANDESK installs using a Local System account to connect to the drive. You might need to grant Domain Computers access to your share (the local system machines connect as Domain Computer if they are in the domain).
I will have a go at changing the package to use a network account as has been suggested above tomorrow before the New York office opens.