Can SQL Server R Services be enabled on a readable secondary?
Readable Secondary is a feature of Always On Availability Groups and you can easily create one of these in Azure using the template from the gallery.

You have to do a little bit of extra config, to configure the read-only routing, something like this:

-- Configure the READ_ONLY_ROUTING_URL
ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-1'
WITH ( SECONDARY_ROLE( READ_ONLY_ROUTING_URL = N'TCP://sqlserver-1.contoso.com:1433') )

ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-0'
WITH ( SECONDARY_ROLE( READ_ONLY_ROUTING_URL = N'TCP://sqlserver-0.contoso.com:1433') )

-- Configure the READ_ONLY_ROUTING_LIST
ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-1' 
WITH (PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(N'sqlserver-0', N'sqlserver-1')))

ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-0' 
WITH (PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(N'sqlserver-1', N'sqlserver-0')))


Connecting to the readable secondary is then a matter of using the ApplicationIntent=ReadOnly facet in your connection string.  I have not tried doing this from R but imagine you could.  There's an easy way to verify whether or not you are connected to the secondary using sqlcmd which I used recently here.

As far as the push compute goes, this is not yet known.  Looking on the Always On Availability Groups: Interoperability R Services is not listed.

I would personally be surprised if these features worked together in that way as R is so new, but it's a good thought.  If it turns out it doesn't work, you could make it a feature request.  I would also say though, when you connect to an AG listener and request read-only routing, it is only that, a request.  As AOAG is a high-availability feature, if the secondary is down, you would connect to the primary.  So let's say you only enabled R Services on the secondary, you create a situation where these scripts failed when the secondary was not available.

Unfortunately I can't test this out myself as I've burned all my Azure credits this month answering some question for @BrentO : )  Maybe later in the month.