Pad left or right with string.format (not padleft or padright) with arbitrary string

Can I use String.Format() to pad a certain string with arbitrary characters?

Console.WriteLine("->{0,18}<-", "hello");
Console.WriteLine("->{0,-18}<-", "hello");

returns 

->             hello<-
->hello             <-

I now want the spaces to be an arbitrary character. The reason I cannot do it with padLeft or padRight is because I want to be able to construct the format string at a different place/time then the formatting is actually executed.

--EDIT--
Seen that there doesn't seem to be an existing solution to my problem I came up with this (after Think Before Coding's suggestion)
--EDIT2--
I needed some more complex scenarios so I went for Think Before Coding's second suggestion

[TestMethod]
public void PaddedStringShouldPadLeft() {
	string result = string.Format(new PaddedStringFormatInfo(), "->{0:20:x} {1}<-", "Hello", "World");
	string expected = "->xxxxxxxxxxxxxxxHello World<-";
	Assert.AreEqual(result, expected);
}
[TestMethod]
public void PaddedStringShouldPadRight()
{
	string result = string.Format(new PaddedStringFormatInfo(), "->{0} {1:-20:x}<-", "Hello", "World");
	string expected = "->Hello Worldxxxxxxxxxxxxxxx<-";
	Assert.AreEqual(result, expected);
}
[TestMethod]
public void ShouldPadLeftThenRight()
{
	string result = string.Format(new PaddedStringFormatInfo(), "->{0:10:L} {1:-10:R}<-", "Hello", "World");
	string expected = "->LLLLLHello WorldRRRRR<-";
	Assert.AreEqual(result, expected);
}
[TestMethod]
public void ShouldFormatRegular()
{
	string result = string.Format(new PaddedStringFormatInfo(), "->{0} {1:-10}<-", "Hello", "World");
	string expected = string.Format("->{0} {1,-10}<-", "Hello", "World");
	Assert.AreEqual(expected, result);
}

Because the code was a bit too much to put in a post, I moved it to github as a gist:
http://gist.github.com/533905#file_padded_string_format_info

There people can easily branch it and whatever :)

There is another solution.

Implement IFormatProvider to return a ICustomFormatter that will be passed to string.Format :

public class StringPadder : ICustomFormatter
{
  public string Format(string format, object arg,
       IFormatProvider formatProvider)
  {
     // do padding for string arguments
     // use default for others
  }
}

public class StringPadderFormatProvider : IFormatProvider
{
  public object GetFormat(Type formatType)
  { 
     if (formatType == typeof(ICustomFormatter))
        return new StringPadder();

     return null;
  }
  public static readonly IFormatProvider Default =
     new StringPadderFormatProvider();
}

Then you can use it like this :

string.Format(StringPadderFormatProvider.Default, "->{0:x20}<-", "Hello");

You could encapsulate the string in a struct that implements IFormattable

public struct PaddedString : IFormattable
{
   private string value;
   public PaddedString(string value) { this.value = value; }

   public string ToString(string format, IFormatProvider formatProvider)
   { 
      //... use the format to pad value
   }

   public static explicit operator PaddedString(string value)
   {
     return new PaddedString(value);
   }
}

Then use this like that :

 string.Format("->{0:x20}<-", (PaddedString)"Hello");

result:

"->xxxxxxxxxxxxxxxHello<-"

Edit: I misunderstood your question, I thought you were asking how to pad with spaces.

What you are asking is not possible using the string.Format alignment component; string.Format always pads with whitespace. See the Alignment Component section of MSDN: Composite Formatting.

According to Reflector, this is the code that runs inside StringBuilder.AppendFormat(IFormatProvider, string, object[]) which is called by string.Format:

int repeatCount = num6 - str2.Length;
if (!flag && (repeatCount > 0))
{
    this.Append(' ', repeatCount);
}
this.Append(str2);
if (flag && (repeatCount > 0))
{
    this.Append(' ', repeatCount);
}

As you can see, blanks are hard coded to be filled with whitespace.

Simple:

dim input as string = "SPQR"
dim format as string =""
dim result as string = ""

'pad left:
format = "{0,-8}"
result = String.Format(format,input)
'result = "SPQR    "

'pad right
format = "{0,8}"
result = String.Format(format,input)
'result = "    SPQR"