Couldnt do this with a list button as that resides on the asset object itself so it wont be able to tell the id of the parent loan record until it has been assigned which kind of negates the purpose I would think. That being said it can be done when referencing a Visualforce page using a detail button. As such here is the code I put in place and tested. 

<!------------Visualforce----------->
<apex:page standardController="Loan__c" extensions="ViewAvailableAssets">
   <apex:form >  
    <apex:pageBlock >
        
            <apex:pageBlockButtons >
                          
                <apex:commandButton action="{!updateAssets}" value="Update Assets"/>                       
                
                </apex:pageBlockButtons>
         
            <apex:pageBlockSection >
                <apex:pageBlockTable title="All Assets" var="a" value="{!allAssets}">
                    <apex:column headerValue="Select">
                        <apex:inputCheckbox value="{!a.selected}" />
                    </apex:column>
                    <apex:column value="{!a.asset.Name}" headerValue="Asset Name"/>
                    
                </apex:pageBlockTable>
                
            
            </apex:pageBlockSection>
        
    </apex:pageBlock>
        </apex:form>  
    
</apex:page>


<!----------End Visualforce----------->

/*******************Apex********************/
public class ViewAvailableAssets {
    public List<Asset> selectedAssets;
    public List<AssetRecord> allAssets{get;set;}
    public ViewAvailableAssets(ApexPages.StandardController controller){
        populateAllAssets();
    }
    
    /**
     * This method populates the allAssets list with
     * objects built in our inner class.
     */
    public void populateAllAssets()
    {
         allAssets = new List<AssetRecord>();
        List<Asset> assets = [Select Name From Asset Where LoanRef__c = null Order By Name];
        for(Asset a: assets){
            allAssets.add(new AssetRecord(a));            
        }
    }
    /**
     * This method is called from a button push on the visualforce page.
     * when called it checks the status of  the selected attribute in in all
     * our AssetRecord objects. If they are true it places a copy of the Asset
     * record associated to that object in our selectedAssets list which is a list
     * of Asset records. It then updates the asset records and redirects to the 
     * loan record that started the process.
     */
    public PageReference updateAssets(){
        selectedAssets  = new List<Asset>();
        String loanId = ApexPages.currentPage().getParameters().get('id');
        Loan__c loan = [Select Id From Loan__c Where Id = :loanId];
        for(AssetRecord ar : allAssets)
        {
            system.debug(ar.selected);
            if(ar.selected)
            {
                Asset asset = ar.asset;
                asset.LoanRef__c = loanId;
                selectedAssets.add(asset);
            }
        }
        update selectedAssets;
        PageReference pr = new ApexPages.StandardController(loan).view();
        pr.setRedirect(true);
        return pr;
    }
    
    /**
     * This inner class is used to hold a single asset record and
     * associate it to a boolean variable, and creates an object of type AssetRecord.
     */
    public class AssetRecord {
        public Boolean selected {get;set;}
        public Asset asset {get;set;}
        /**
         * Constructor Method.
         * sets the objects variables.
         * @param asset an Asset Record
         */ 
        public AssetRecord(Asset asset){
            selected = false;
            this.asset= asset;
        }
    }
}


/*******************End Apex********************/
When a the detail button is pushed from the Loan Object


The visualforce page opens loading all the assets that do not have a loan assigned to them.
Select the assets that will be assigned and click the "Update Assets" button.


The assets will be assigned to the loan and the page will redirect back to the loan record.