Draw XS From Site Model without Base Object

Not directly.  And as you noted, sometimes the Object+slopes option is not adequate.

Here is an easy suggestion that might work for you.

Instead of using the model option use the following:

• Object + slopes for the main object
• Object (no slopes) for the islands.  Just add each of your islands to the list of surfaces.

This will give the result you need which is main object and tie slopes and also show the islands or other objects which lie entirely within the main object.  Two possible drawbacks: 

1. An island will certainly fall within the limits of a road object.  But there might be a situation where you have two object adjacent (or overlapping only partly) where you need side slopes from both objects
2. At your islands there will be dual linework.  One for top of roadway and another for top of island, instead of a single line following the ultimate model finish grade.

I have found the above method to appropriate most of the time but if not then there is a pretty straitforward procedure that might be useful 

1. Use the model as you have been doing.  As you know this makes a model line which duplicates the existing ground for much of the section.
2. Use power selector and select all the proposed ground lines and drop them from linestrings to lines.  (If the proposed ground is on a seperate level and/or color will help)
3. Run the data cleanup utility (Utilities > Data Cleanup) and use the "Overlaps" tab. 
   1. Set the action to select so you get a chance to review the results before deleting
   2. Turn Off the Match items for level, color, style, weight and user data
   3. Click Apply.  This should select all the parts of proposed ground that overlap (lie on top of) the existing ground
   4. Once Selected and you review the selection you can delete these overlapping parts by pressing the delete key.

FYI - I posted an enhancement request to allow the model option to clip at the juntion with existing ground which has not been implemented yet.  As always, if enhancements like this are important to you then filing a change request of your own is very important.

Not directly.  And as you noted, sometimes the Object+slopes option is not adequate.

Here is an easy suggestion that might work for you.

Instead of using the model option use the following:

• Object + slopes for the main object
• Object (no slopes) for the islands.  Just add each of your islands to the list of surfaces.

This will give the result you need which is main object and tie slopes and also show the islands or other objects which lie entirely within the main object.  Two possible drawbacks: 

1. An island will certainly fall within the limits of a road object.  But there might be a situation where you have two object adjacent (or overlapping only partly) where you need side slopes from both objects
2. At your islands there will be dual linework.  One for top of roadway and another for top of island, instead of a single line following the ultimate model finish grade.

I have found the above method to appropriate most of the time but if not then there is a pretty straitforward procedure that might be useful 

1. Use the model as you have been doing.  As you know this makes a model line which duplicates the existing ground for much of the section.
2. Use power selector and select all the proposed ground lines and drop them from linestrings to lines.  (If the proposed ground is on a seperate level and/or color will help)
3. Run the data cleanup utility (Utilities > Data Cleanup) and use the "Overlaps" tab. 
   1. Set the action to select so you get a chance to review the results before deleting
   2. Turn Off the Match items for level, color, style, weight and user data
   3. Click Apply.  This should select all the parts of proposed ground that overlap (lie on top of) the existing ground
   4. Once Selected and you review the selection you can delete these overlapping parts by pressing the delete key.

FYI - I posted an enhancement request to allow the model option to clip at the juntion with existing ground which has not been implemented yet.  As always, if enhancements like this are important to you then filing a change request of your own is very important.